Musnad Ahmad 26:505

Sulayman ibn Dawud narrated to us, Zuhayr narrated to us, Abu Ishaq narrated to us, from Abd al-Rahman ibn al-Aswad, from Alqamah and al-Aswad, from Abdullah, who said: I saw the Messenger of Allah (may Allah bless him and grant him peace) 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 the side of his cheek became visible. And I saw Abu Bakr and Umar doing that.