Musnad Ahmad 26:189

Abu Kamil narrated to us, Zuhayr narrated to us, Abu Ishaq narrated to us, from Abd al-Rahman ibn al-Aswad, from al-Aswad and Alqamah, from Abdullah, who said: I saw the Prophet, peace and blessings of Allah be upon him, saying the takbir upon every rising and lowering, and standing and sitting. And he would say the taslim to his right and to his left: "Peace be upon you and the mercy of Allah, peace be upon you and the mercy of Allah," until I could see the whiteness of his cheek. And I saw Abu Bakr and Umar doing that.