The obligation of the final tashahhud

Sayyiduna Ibn Abbas (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) used to teach us the tashahhud just as he taught us the Qur'an, and he would say: "'At-tahiyyatul-mubarakatus-salawatut-tayyibatu lillah, salamun alayka ayyuhan-nabiyyu wa rahmatullahi wa barakatuh, salamun alayna wa ala ibadillahis-salihin, ashhadu an la ilaha illallah, wa ashhadu anna Muhammadan rasulullah' (All blessed verbal, physical and financial acts of worship are for Allah. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us and upon all the righteous servants of Allah. I bear witness that there is no deity worthy of worship except Allah, and I bear witness that Muhammad (peace and blessings be upon him) is the Messenger of Allah)."

It is narrated from Ibn Abbas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to teach us the tashahhud just as he would teach us a surah of the Qur'an.

Hittan bin Abdullah al-Raqashi narrates that Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) led the people in prayer; he then mentioned the complete hadith. In it, he (peace and blessings be upon him) said: "When he sits, the first thing he should say is: 'At-tahiyyatu at-tayyibatu az-zakiyatu lillah, as-salamu 'alayka ayyuha an-Nabiyyu wa rahmatullahi wa barakatuh, as-salamu 'alayna wa 'ala 'ibadillahi as-salihin, ashhadu an la ilaha illallah, wa ashhadu anna Muhammadan 'abduhu wa rasuluh' (All verbal, physical and financial acts of worship are for Allah. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us and upon all the righteous. I bear witness that there is no god but Allah, and I bear witness that Muhammad (peace and blessings be upon him) is the servant of Allah and His Messenger)."

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: When we used to pray, we did not know what we should say. We used to say: As-salamu 'ala Allah, as-salamu 'ala Jibra'il, as-salamu 'ala Mika'il, as-salamu 'ala an-nabiyyin (Peace be upon Allah, peace be upon Jibra'il, peace be upon Mika'il, peace be upon the prophets). Then the Messenger of Allah (peace and blessings be upon him), teaching us the salam, said: "Indeed, Allah Himself is al-Salam (Peace). So when you pray, say: 'At-tahiyyatu lillahi was-salawatu wat-tayyibat, as-salamu 'alayka ayyuha an-Nabiyyu wa rahmatullahi wa barakatuh, as-salamu 'alayna wa 'ala 'ibadillahi as-salihin' (All verbal, physical and financial acts of worship are for Allah. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us and upon all the righteous)."
Abu Wa'il (may Allah have mercy on him), in his narration through Abdullah (may Allah be pleased with him), reports that he (peace and blessings be upon him) said: "When you say this, the greeting of peace reaches every angel brought near and every messenger, and every righteous servant as well. 'Ashhadu an la ilaha illallah wa ashhadu anna Muhammadan 'abduhu wa rasuluh' (I bear witness that there is no god but Allah, and I bear witness that Muhammad (peace and blessings be upon him) is the servant of Allah and His Messenger)."

Besides this, a similar hadith is narrated through another chain.

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: Before the tashahhud was made obligatory upon us, we used to say: 'As-salamu 'ala Allahi qabla khalqih' (Peace be upon Allah before His creation), peace be upon Jibra'il and Mika'il. Then the Messenger of Allah (peace and blessings be upon him) taught us the tashahhud.

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) used to teach us the tashahhud just as he would teach us a surah of the Qur'an, and he would say: "There is no prayer without the tashahhud."
With its sound supporting narrations (shawahid), this hadith gains some degree of strength.

(a) Abdullah (may Allah be pleased with him) narrates: The tashahhud is what completes the prayer.
(b) It has been narrated to us from Umar bin al-Khattab (may Allah be pleased with him) that he said: No prayer is valid without the tashahhud.