Musannaf Ibn Abi Shayba 3:609

Abu Nu'aym narrated to us, he said: Sayf ibn Sulayman narrated to us, he said: I heard Mujahid say: 'Abd Allah ibn Sakhbara narrated to me, he said: I heard Ibn Mas'ud say: The Messenger of Allah (may Allah bless him and grant him peace) taught me the Tashahhud, [with] my palm between his palms, just as he would teach me a Surah from the Qur'an: "At-Tahiyyatu lillahi, was-salawatu, wat-tayyibatu. As-salamu 'alayka ayyuha an-nabiyyu wa rahmatullahi wa barakatuhu. As-salamu 'alayna wa 'ala 'ibadillahi as-salihin. Ashhadu an la ilaha illallahu wa ashhadu anna Muhammadan 'abduhu wa rasuluhu." And he was among us. Then, when he was taken [passed away], we said: "As-salamu 'ala an-nabiyyi" (Peace be upon the Prophet).