Why is Ibraaheem (peace be upon him) singled out for mention in the Tashahhud?

Allah, may He be
glorified and exalted, singled out Sayyiduna Ibraaheem (blessings and peace
of Allah be upon him) for great favours and honour. He was described in the
Qur’an as an imam (leader), an ummah (a nation in himself), a haneef
(monotheist),  and devoted to Allah, may He be glorified and exalted; all
the Prophets who came after him were descended from him and the followers of
all religions (Muslims, Christians and Jews) believe in him. 

Ibraaheem (peace
be upon him) is the best of the Prophets and Messengers after our Prophet
Muhammad (blessings and peace of Allah be upon him). Hence Allah has told us
that He took him as a close friend. “And
Allah did take Ibrahim (Abraham) as a Khaleel (an intimate friend)”
[an-Nisa’ 4:125]. 

All the Prophets
who came after him are descended from him via Ishaaq and Ya‘qoob, apart from
Muhammad (blessings and peace of Allah be upon him), who is descended from
Ismaa‘eel ibn Ibraaheem. 

Our Prophet
(blessings and peace of Allah be upon him) is closer to Ibraaheem than
anyone else. So Ibraaheem (peace be upon him) is the father of the Arabs and
the forefather of the Prophet (blessings and peace of Allah be upon him) in
terms of descent. 

Ibraaheem is the
one whom the Prophet (blessings and peace of Allah be upon him) instructed
his community to follow. “Then, We have inspired you (O Muhammad SAW
saying): “Follow the religion of Ibraheem (Abraham) Haneefa (Islamic
Monotheism – to worship none but Allah)” [an-Nahl 16:123].

Hence our Prophet
(blessings and peace of Allah be upon him) is the closest of people to
Ibraaheem (peace be upon him), and so are we, as his followers. 

Allah, may He be
glorified and exalted, says (interpretation of the meaning):

“Verily,
among mankind who have the best claim to Ibraheem (Abraham) are those who
followed him, and this Prophet (Muhammad SAW) and those who have believed
(Muslims)”

[Aal ‘Imraan
3:68].

And He says,
refuting the Jews and the Christians:

“Ibraheem
(Abraham) was neither a Jew nor a Christian, but he was a true Muslim
Haneefa (Islamic Monotheism – to worship none but Allah Alone) and he was
not of Al-Mushrikoon”

[Aal ‘Imraan
3:67]. 

Shaykh Ibn
‘Uthaymeen (may Allah have mercy on him) was asked: Why is Ibraaheem (peace
be upon him) singled out for the call of Tawheed, when all the Prophets
called people to Tawheed (belief in the Oneness of Allah)? 

He replied: 

All the Prophets
brought the message of Tawheed. Allah, may He be exalted, says
(interpretation of the meaning):

“And
We did not send any Messenger before you (O Muhammad SAW) but We inspired
him (saying): La ilaha illa Ana (none has the right to be worshipped but I
(Allah)), so worship Me (Alone and none else)”

[al-Anbiya’
21:25].

But Ibraaheem was
the father of the Arabs, and the father of the Israelites, and he called
people to pure Tawheed. The Jews and the Christians claimed to be his
followers, but the Muslims are the ones who are his followers. He (peace and
blessings be upon him) was chosen to be the father of the Prophets; he is
the promoter of pure monotheism and we are instructed to follow him, because
we are closer to Ibraaheem, as Allah, may He be exalted, says
(interpretation of the meaning):

“Verily,
among mankind who have the best claim to Ibraheem (Abraham) are those who
followed him, and this Prophet (Muhammad SAW) and those who have believed
(Muslims)”

[Aal ‘Imraan
3:68].

And He says,
refuting the Jews and the Christians:

“Ibraheem
(Abraham) was neither a Jew nor a Christian, but he was a true Muslim
Haneefa (Islamic Monotheism – to worship none but Allah Alone) and he was
not of Al-Mushrikoon”

[Aal ‘Imraan
3:67]. 

Liqa’ al-Baab
al-Maftooh,
189/question no. 7 

Al-‘Allaamah Badr
ad-Deen al-‘Ayni al-Hanafi (may Allah have mercy on him) noted another point
with regard to this issue, as he said: 

If it is said: Why
was Ibraaheem (peace be upon him) singled out from among all the other
Prophets (peace be upon them) to be mentioned during the prayer? 

I say: That is
because on the night of the Mi‘raaj, the Prophet (blessings and peace of
Allah be upon him) saw all the Prophets and Messengers, and he greeted each
Prophet with salaam, but none of them sent greetings to his ummah apart from
Ibraaheem (peace be upon him). Hence the Prophet (blessings and peace of
Allah be upon him) instructed us to send blessings upon him at the end of
every prayer until the Day of Resurrection, in response to his kindness. 

It was also said
that when Ibraaheem (peace be upon him) had finished building the Ka‘bah, he
prayed for the ummah of Muhammad (blessings and peace of Allah be upon him),
saying: O Allah, whoever performs pilgrimage to this House from the ummah of
Muhammad, give him greetings from me; and his family and children also
offered this supplication, so we are instructed to remember them in the
prayer, in response to their kind deed. End quote. 

Sharh Sunan Abi
Dawood
by al-‘Ayni, 4/260 

And Allah knows
best.