Musnad Abu Ya'la al-Mawsili 152:266

Abu Khaythama narrated to us, [he said] al-Hasan ibn Musa and Humayd ibn Abd al-Rahman narrated to us, they said: Zuhayr narrated to us, from Abu Ishaq, from Abd al-Rahman ibn al-Aswad, from his father and Alqama, from Abdullah, who said: I saw the Messenger of Allah (may Allah bless him and grant him peace) saying the takbir upon every rising, lowering, and standing. 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 cheeks. And I saw Abu Bakr and Umar doing that.