Musnad Ahmad 26:113

Yahya narrated to us, from Zuhayr, who said: Abu Ishaq narrated to me, from Abd al-Rahman ibn al-Aswad, from al-Aswad and Alqama, from Abdullah, who said: I saw the Messenger of Allah (may Allah bless him and grant him peace) saying the takbir in every lowering and rising, and standing and sitting. And he would say the taslim to his right and to his left, until the whiteness of his cheeks—or his cheek—could be seen. And I saw Abu Bakr and Umar doing that.