Musnad Ahmad 27:953

Abu Salamah narrated to us, Abd al-Aziz ibn Muhammad ibn al-Darawardi, the freed slave of Banu Layth, informed us, from Amr ibn Yahya ibn Umarah ibn Abi Hasan al-Ansari, then al-Muharibi, from Muhammad ibn Yahya ibn Habban, from his paternal uncle Wasi ibn Habban, who said: I said to Ibn Umar: "Inform me about the prayer of the Messenger of Allah, peace and blessings of Allah be upon him, how was it?" He said: "He mentioned the takbir (saying Allahu Akbar), every time he lowered his head and every time he raised it, and he mentioned: 'As-salamu alaykum wa rahmatullah' to his right, and 'As-salamu alaykum' to his left."