Should He Forsake His Family Who Reject the Sunnah?

Firstly: 

Every Muslim has to believe in all the hadeeths of the
Messenger (peace and blessings of Allah be upon him) – if they are saheeh
(authentic) – and not reject any of them, because his hadeeths and his
Sunnah (teachings) are revelation (wahy) from Allah. Whoever rejects the
hadeeth of the Messenger of Allah (peace and blessings of Allah be upon him)
has rejected revelation from Allah. 

Allah says (interpretation of the meaning): 

“By the star when it goes down (or vanishes).

Your companion (Muhammad) has neither gone astray nor has
erred.

Nor does he speak of (his own) desire.

It is only a Revelation revealed.

He has been taught (this Quran) by one mighty in power
(Gabriel).

One free from any defect in body and mind then he (Gabriel in
his real shape as created by Allah) rose and became stable.” [53:1-6] 

Allah has commanded the people to obey His Prophet (peace and
blessings of Allah be upon him). He has enjoined this in many verses of the
Quran, of which we will quote some. Allah says (interpretation of the
meaning): 

“Say (O Muhammad): “Obey Allah and the Messenger (Muhammad).”
But if they turn away, then Allah does not like the disbelievers.” [3:32] 

“He who obeys the Messenger (Muhammad), has indeed obeyed
Allah, but he who turns away, then we have not sent you (O Muhammad) as a
watcher over them.” [4:80] 

“O you who believe! Obey Allah and obey the Messenger
(Muhammad), and those of you (Muslims) who are in authority. (And) if you
differ in anything amongst yourselves, refer it to Allah and His Messenger,
if you believe in Allah and in the Last Day. That is better and more
suitable for final determination.” [4:59] 

“And perform As‑Salah (Iqamat‑as-Salah), and give Zakah and
obey the Messenger (Muhammad) that you may receive mercy (from Allah).”
 [24:56] 

And there are many similar verses. 

The one who rejects the Sunnah is a kafir and an apostate. 

Al-Suyooti (may Allah have mercy on him) said in his essay
Miftah al-Jannah fi Ihtijaj bi’l-Sunnah: 

Note that whoever denies that the hadeeth of the Prophet
(peace and blessings of Allah be upon him), whether it describes his words
or deeds, so long as it meets the conditions outlined by the scholars, may
be quoted as evidence, is a kafir and has gone beyond the pale of Islam; he
will be gathered with the Jews and the Christians or whomever Allah wills
among the kafir sects [i.e., on the Day of Resurrection]. 

Those who want to restrict themselves to the Quran only are
called al-Quraniyyoon. This view of theirs is an old view which the
Messenger (peace and blessings of Allah be upon him) warned against in more
than one hadeeth, as we shall see below. Among the soundest evidence that
this view is false is the fact that those who say this do not really follow
what they say. 

How do these people pray? How many times do they pray each
day and night? What are the conditions and details of zakah? What is the
nisab (threshold of wealth) for paying zakah? What is the amount that must
be paid? How do they do Hajj and ‘Umrah? How many times do they
circumambulate the Ka’bah? How many times do they go back and forth between
al-Safa and al-Marwah? 

There are many other issues for which the details are not
narrated in the Quran, rather they are mentioned in the Quran in general
terms, and the Messenger (peace and blessings of Allah be upon him)
explained them in detail in his Sunnah. 

Would these people refrain from acting upon these rulings
because they are not narrated in the Quran? 

If their answer is yes, then they have passed judgement
against themselves that they are kafirs, because they have denied a basic
principle of Islam that no Muslim has any excuse for not knowing and on
which there is unanimous consensus among the Muslims.  

If they reply that they do not refrain from following these
rulings, then they have demonstrated that their view is false. 

Shaykh al-Islam Ibn Taymiyah said – after quoting the verses
that enjoin following the Messenger (peace and blessings of Allah be upon
him) –  

These texts enjoin following the Messenger (peace and
blessings of Allah be upon him) even if we do not find what he said
specifically referred to in the texts of the Quran. These verses also enjoin
following the Quran even if we do not find what is said in the Quran
specifically mentioned in the hadeeth of the Messenger (peace and blessings
of Allah be upon him). 

So we must follow the Quran and we must follow the Messenger
(peace and blessings of Allah be upon him). Following the one implies
following the other, for the Messenger (peace and blessings of Allah be upon
him) conveyed the Book, and the Book commands us to follow the Messenger
(peace and blessings of Allah be upon him). The Book and the Messenger
(peace and blessings of Allah be upon him) do not contradict one another at
all, just as the Book does not contradict itself. Allah says (interpretation
of the meaning): 

“Had it been from other than Allah, they would surely, have
found therein many a contradiction.” [4:82] 

And there are many hadeeths from the Prophet (peace and
blessings of Allah be upon him) which state that it is obligatory to follow
the Quran and that it is obligatory to follow his Sunnah, for example, the
hadeeth in which he said: “I do not want to find any one of you reclining on
his pillow, and when he hears of something that I have commanded or
forbidden, he says, ‘Between us and you there stands this Quran, whatever we
find is permissible in it we will take as permissible, and whatever we find
is forbidden in it we will take as forbidden.’ For I have been given the
Book and something like it with it; it is like the Quran or more.” This
hadeeth is narrated in the books of Sunan and Musnad from the Prophet (peace
and blessings of Allah be upon him) with a number of isnads (chains of
narration) from Abu Tha’labah, Abu Rafi’, Abu Hurayrah and others. 

In Saheeh Muslim is it narrated from Jabir that the Prophet
(peace and blessings of Allah be upon him) said in his Farewell Sermon: “I
am leaving behind among you something which, if you adhere to it, you will
not go astray after that. It is the Book of Allah.” The version narrated by
al-Hakim says “The Book of Allah and my Sunnah.” (Classed as
saheeh/authentic by al-Albani in Saheeh al-Jami’, 2937). 

In al-Saheeh it is narrated from ‘Abd-Allah ibn Abi Awfa that
it was said to him: Did the Messenger of Allah leave a will? He said, No. It
was said, How then is it prescribed for people to make wills when the
Prophet (peace and blessings of Allah be upon him) did not make a will? He
said, He made a will in which he enjoined (adherence to) the Book of Allah. 

(Narrated by Muslim, 1634) 

The Sunnah of the Messenger of Allah explains the Quran, as
it explains the number of prayers, how much should be recited in them, which
should be recited out loud and which quietly. It also explains the amounts
of zakah to be paid and the threshold at which zakah becomes due; the
rituals of Hajj and ‘Umrah; how many times one should circumambulate the
Ka’bah, go between al-Safa and al-Marwah and stone the Jamarat (pillars),
etc. 

When any Sunnah is proven to be saheeh, the Muslims are
agreed that it is obligatory to follow it. There may be something in the
Sunnah which a person thinks appears to go against the apparent meaning of
the Quran and add to it, such as the Sunnah which explains the threshold of
stealing at which the hadd (Islamic legal) punishment becomes due, and the
Sunnah which stipulates that the married adulterer is to be stoned. This
Sunnah must also be followed, according to the view of the
Sahabah/Companions (may Allah be pleased with them) and those who followed
them in truth, and all the groups of Muslims. 

Adapted from Majmoo’ al-Fatawa, 19/84-86 

That which was brought by the Messenger (peace and blessings
of Allah be upon him) is true just as the Quran is true.

 Secondly: 

You should not forsake your family; rather you should treat
them kindly and strive to call them to follow and accept the Sunnah. 

Allah says (interpretation of the meaning): 

“Invite (mankind, O Muhammad) to the way of your Lord (i.e.
Islam) with wisdom (i.e. with the Divine Revelation and the Quran) and fair
preaching, and argue with them in a way that is better. Truly, your Lord
knows best who has gone astray from His path, and He is the Best Aware of
those who are guided.” [16:125] 

“And We have enjoined on man (to be dutiful and good) to his
parents. His mother bore him in weakness and hardship upon weakness and
hardship, and his weaning is in two years; give thanks to Me and to your
parents. Unto Me is the final destination.

But if they (both) strive with you to make you join in
worship with Me others that of which you have no knowledge, then obey them
not; but behave with them in the world kindly, and follow the path of him
who turns to Me in repentance and in obedience. Then to Me will be your
return, and I shall tell you what you used to do.” [31:14-15] 

And Allah knows best.