Musnad Ahmad 26:385

Abu Nu'aym narrated to us, Sayf narrated to us, he said: I heard Mujahid saying: 'Abdullah ibn Sakhbarah, Abu Ma'mar, narrated to me, he said: I heard Ibn Mas'ud saying: The Messenger of Allah (ﷺ) taught me the Tashahhud—my palm between his palms—just as he would teach me a Surah from the Qur'an. He said: "The greetings (at-tahiyyat) are for Allah, and the prayers (as-salawat) and the good things (at-tayyibat). 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. I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and His messenger." This was while he was among us. But when he passed away, we said: "Peace be upon the Prophet."