Musnad Abu Ya'la al-Mawsili 152:60

Muhammad ibn Abi Bakr narrated to us, Yahya and Mu'adh ibn Mu'adh narrated to us, from Zuhayr, from Abu Ishaq, from 'Abd al-Rahman ibn al-Aswad, from his father and 'Alqamah, from 'Abdullah: The Prophet, peace and blessings of Allah be upon him, would say the takbir during every lowering, rising, standing, and sitting. He would perform the taslim to his right and to his left: "As-salamu 'alaykum wa rahmatullah, as-salamu 'alaykum wa rahmatullah," until the whiteness of his cheek could be seen. Abu Bakr and 'Umar used to do that.