Musnad Ahmad 26:456

Yahya ibn Adam narrated to us, Zuhayr narrated to us, al-Hasan ibn al-Hurr narrated to us, he said: al-Qasim ibn Mukhaymarah narrated to me, he said: Alqamah took my hand and narrated to me that Abdullah ibn Mas’ud took his hand, and that the Messenger of Allah (may Allah bless him and grant him peace) took the hand of Abdullah, and he taught him the Tashahhud in the prayer. He said: "Say: The greetings are for Allah, and the prayers, and the good things. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us and upon the righteous servants of Allah — Zuhayr said: I memorized from him, if Allah wills — I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and His messenger." He said: "So when you have finished this," or he said: "So when you have done this, you have completed your prayer. If you wish to stand, then stand, and if you wish to sit, then sit."