Does the disbeliever have to shave his head if he becomes Muslim?

Firstly: 

It is narrated in a number of hadiths that the disbeliever is
to be instructed to shave his head if he becomes Muslim, but none of these
hadiths are saheeh. 

Imam ‘Abd ar-Razzaaq as-San‘aani said in his Musannaf
(10/317): Ibn Jurayj told us: I was informed, from ‘Uthaym ibn Kulayb, from
his father, from his grandfather, that he came to the Prophet (blessings and
peace of Allah be upon him) and said: I have become Muslim, and the Prophet
(blessings and peace of Allah be upon him) said: “Shave off the hair of
disbelief.” 

It was narrated via ‘Abd ar-Razzaaq by Imam Ahmad in his
Musnad (15432) and by Abu Dawood in his Sunan (356). 

This isnaad is da‘eef jiddan (very weak), because it includes
three unknown narrators: the Shaykh of Ibn Jurayj, who is not named in the
isnaad, ‘Uthaym ibn Katheer and his grandfather Katheer ibn Kulayb. 

In fact it was said that the shaykh of Ibn Jurayj in this
isnaad is Ibraaheem ibn Abi Yahya, whose hadith is to be rejected and who
was subject to criticism by the majority of muhadditheen. 

Please see: al-Jarh wa’t-Ta‘deel (2/125) and
Tahdheeb at-Tahdheeb (1/158) 

Ibn al-Qayyim said: With regard to this Ibraaheem, there was
consensus among the hadith scholars that he is weak – apart from ash-Shaafa‘i
only. 

End quote from Tuhfat al-Mawdood (p. 170) 

Ibn ‘Adiyy said: What Ibn Jurayj said in this isnaad, “I was
informed, from ‘Uthaym ibn Kulayb” means that Ibraaheem ibn Abi Yahya is the
one who narrated it to him, and he mentioned him by his kunyah, not his full
name. 

End quote from al-Kaamil fi Du‘afa’ ar-Rijaal (1/361) 

Ibn Taahir al-Maqdisi said:

The man whom he mentioned by his kunyah is this Ibraaheem,
who is da‘eef jiddan (very weak), and he narrated it from ‘Uthaym ibn
Katheer ibn Kulayb. 

End quote from Dhakheerat al-Huffaaz (1/457) 

Ibn al-Qattaan said: Its isnaad is extremely weak, in
addition to the interruption indicated by the words of Ibn Jurayj, “I was
informed.” That is because ‘Uthaym ibn Kulayb and his father and grandfather
are majhool (unknown). 

Would that the matter stopped there, but there are further
reservations about this hadith because some of the hadith scholars said:
With regard to Ibn Jurayj, who said “I was informed from ‘Uthaym ibn Kulayb”,
it was only narrated to him from ‘Uthaym ibn Kulayb by Ibraaheem ibn Abi
Yahya, who is known to be da‘eef, and there were other concerns regarding
his religious commitment. 

End quote from Bayaan al-Wahm wa’l-Eehaam (3/43). 

The commentators on Musnad al-Imam Ahmad said: ‘Uthaym
ibn Kulayb was named after his grandfather, whose full name was ‘Uthaym ibn
Katheer ibn Kulayb al-Hadrami. Many people narrated from him, and he was
mentioned by Ibn Hibbaan in ath-Thiqaat. Adh-Dhahabi said in al-Kaashif:
He was trustworthy. Al-Haafiz said in at-Taqreeb: He is majhool
(unknown). We have not come across any biographical details of his father,
but the rest of its men are thiqaat (trustworthy). End quote. 

There is corroborating evidence for this hadith in the hadith
of Waathilah ibn al-Asqa‘, which was narrated by at-Tabaraani in al-Mu‘jam
as-Sagheer (2/117) and al-Mu‘jam al-Kabeer (199) via Mansoor ibn
‘Ammaar: My father told us: Ma‘roof Abu’l-Khattaab told us: from Waathilah
ibn al-Asqa‘: When I became Muslim, I came to the Prophet (blessings and
peace of Allah be upon him) and he said to me: “Do ghusl with water and
lotus leaves, and shave off the hair of kufr (disbelief).” 

At-Tabaraani said: Nothing was narrated from Waathilah ibn
al-Asqa‘ except via this isnaad, which was narrated only by Mansoor ibn
‘Ammaar. End quote. 

Al-Haythami said: Its isnaad includes Mansoor ibn ‘Ammaar the
preacher, who is da‘eef (weak).

End quote from Majma‘ az-Zawaa’id (1/283) 

This report was also classed as da‘eef by al-Haafiz ibn Hajar
in al-Talkhees al-Habeer (2/168) 

There is a second corroborating report that was narrated by
at-Tabaraani in al-Mu‘jam al-Kabeer (19/14) via Qataadah ibn al-Fadl
ibn Qataadah ar-Rahaawi, from his father: the paternal uncle of my father,
Haashim ibn Qataadah ar-Rahaawi, told me, that his father said: I came to
the Messenger of Allah (blessings and peace of Allah be upon him) and became
Muslim. 

He said to me: “O Qataadah, do ghusl with water and lotus
leaves, and shave off the hair of kufr (disbelief).” 

Al-Haythami said: It was narrated by at-Tabaraani in al-Kabeer,
and its men are thiqaat (trustworthy).

End quote from Majma‘ az-Zawaa’id (1/283( 

It was classed as da‘eef by al-Haafiz ibn Hajar in at-Talkhees
(2/168) 

Shaykh al-Albaani said: It is clear to me that al-Haafiz was
correct in classing its isnaad as da‘eef, and that his shaykh al-Haythami
was mistaken in classing its men as trustworthy, because in doing that he
relied on Ibn Hibbaan, who narrated all of that from Haashim ibn Qataadah
ar-Rahaawi and al-Fadl ibn Qataadah ar-Rahaawi in his book ath-Thiqaat
(5/503 and 7/317). 

It is well known that Ibn Hibbaan was lenient in classing
narrators as trustworthy, especially when these two men are only known in
this isnaad.

End quote from Silsilat al-Ahaadeeth as-Saheehah
(6/1181). 

However, Shaykh al-Albaani (may Allah have mercy on him)
favoured the view that this hadith is hasan, based on these corroborating
reports. 

But what appears to be the case is that the view of al-Haafiz
ibn Hajar and other scholars, that this hadith is to be classed as da‘eef,
is more likely to be correct, as its isnaads are not free of weakness and
cannot be strengthened. 

Among the prominent scholars who classed this hadith as
da‘eef are:

Ibn Taahir al-Maqdisi, in Dhakheerat al-Huffaaz
(1/457)

An-Nawawi in al-Majmoo‘ (2/154)

Ibn Daqeeq al-Eid in al-Imam (1/417)

Adh-Dhahabi in Tanqeeh at-Tahqeeq (2/264)

Al-Haafiz Ibn Katheer in Irshaad al-Faqeeh (1/34)

Al-Haafiz Ibn Hajar in at-Talkhees (2/168)

As-Suyooti in al-Jaami‘ as-Sagheer (1580)

Al-Mubaarakfoori in Tuhfat al-Ahwadhi (2/529)

Ash-Shawkaani in al-Fath ar-Rabbaani (9/4507) 

Secondly: 

The majority of scholars regard it as mustahabb for the
disbeliever, when he becomes Muslim, to shave his head. See: al-Mawsoo‘ah
at-Fiqhiyyah (18/101) 

As-Sindi said: 

They interpreted this command as meaning that it is mustahabb
(recommended) and said: It is recommended, when the disbeliever becomes a
Muslim, to remove his hair by shaving or cutting it, but shaving it is
preferable.

End quote from Haashiyat Musnad al-Imam Ahmad (8/292) 

An-Nawawi said:

It is recommended for the disbeliever, when he becomes
Muslim, to shave off the hair of his head. This was stated by ash-Shaafa‘i
in al-Umm. End quote from al-Majmoo‘ Sharh al-Muhadhdhab
(2/154) 

In fact some of them said that it is recommended to shave off
all hair on the  head and body, except the beard. 

In Haashiyat al-Bujayrami ‘ala al-Khateeb (1/253) it
says:

It is Sunnah for him also to remove all hair from his body,
including the head and otherwise, because of the report narrated by Abu
Dawood: “Remove the hair of disbelief” – except the beard of the male. End
quote. 

Ibn Qudaamah said:

It is recommended for him to remove his hair, because the
Prophet (blessings and peace of Allah be upon him) instructed a man who
became Muslim: “Shave your hair…” At the very least, this command indicates
that it is recommended (mustahabb). 

End quote from al-Mughni (1/153) 

In Kashshaaf al-Qinaa‘ (1/153) it says:

It is Sunnah to remove his hair, so the head should be
shaved, if he is a man, and the pubic hair and armpit hair should be removed
in all cases. End quote. 

Some of the scholars are of the view that what is meant in
this hadith – assuming that it is saheeh – is shaving off hair in cases
where it is worn in a style that is unique to the disbelievers; hence it is
called in the report “the hair of disbelief”. 

Al-Qarraafi said:

What it refers to is that which is a style that is symbolic
of disbelief; otherwise the people used to enter the religion of Allah in
crowds without shaving their heads.

End quote from adh-Dhakeerah (1/305) 

Al-Mubaarakfoori said:

What is meant by the hair of disbelief is hair worn in a
style that was a sign of disbelief for the disbelievers. Such styles vary
from one land to another. 

The disbelievers of India and Egypt had a spot on the head
where the hair would grow long and they would never cut it or shave it; if
they wanted to shave their head, they would shave all of it except that
spot.

End quote from Tuhfat al-Ahwadhi (3/183) 

It says in ‘Awn al-Ma‘bood (2/15):

It does not mean – and Allah knows best – that everyone who
becomes Muslim should shave his head, in the sense that shaving the head is
required just as ghusl is required. Rather the fact that the hair is
mentioned in conjunction with disbelief (“the hair of disbelief”) indicates
that the hair should be shaved off if it is worn in a style that is a sign
of disbelief for the disbelievers… According to the apparent meaning, this
refers to a sign that distinguishes between disbelief and Islam. End quote. 

Ash-Shawkaani said:

There is no report to suggest that the Prophet (blessings and
peace of Allah be upon him) instructed anyone who became Muslim, among the
senior Sahaabah, to shave off his hair, or that he issued such instructions
to any of those who became Muslim later on, apart from this man.
Nevertheless, the hadith mentioned which speaks of shaving the head is
da‘eef, as the scholars have explained. 

End quote from al-Fath ar-Rabbaani (9/4507) 

With regard to the apostate, we have not come across any
scholar who stated that it is recommended for him to shave off his hair if
he becomes Muslim again. We have not come across anything in the reports
either to suggest that if the apostate becomes Muslim again, he is to be
instructed to shave off his hair. 

What appears to be the case is that he is different from the
one who was originally a disbeliever, despite what we have mentioned about
the difference of opinion concerning the one who was originally a
disbeliever. 

Perhaps if it is said that if an apostate remains an apostate
for a long time, during which his hair grows and can be described as being
the hair of disbelief, or his hair is worn in a style that is a symbol of
disbelief, as is mentioned with regard to the symbols of the disbeliever –
if it is said that in such a case he should be instructed to shave off his
hair, like the one who was originally a disbeliever, and unlike the one
whose case is not like that, who has been an apostate for only a short time
– perhaps if we say this, there may be some validity in this point of view,
in sha Allah. 

Al-‘Ayni said:

The Prophet (blessings and peace of Allah be upon him) only
issued instructions to shave off excess hair in order to clean it and remove
the hair that grew during the time of disbelief.

End quote from Sharh Sunan Abi Dawood (2/183) 

Conclusion: 

There is no saheeh report from the Prophet (blessings and
peace of Allah be upon him) to suggest that the one who becomes Muslim
should be instructed to shave his head. The hadiths that speak of that are
not free of weakness, even though some later scholars classed them as hasan. 

The majority of scholars regard it as recommended for the one
who becomes Muslim to shave his head, and some of them are of the view that
this ruling only applies to the one whose hair is worn in a style that is
exclusive to the disbelievers. In this case – only – he is to be instructed
to shave it off. This view is most likely to be correct. 

For more information, please see the answer to question no.
14051. 

And Allah knows best.