Musnad Abu Ya'la al-Mawsili 154:134

Zuhayr narrated to us, Rawh ibn ‘Ubadah narrated to us, Ibn Jurayj narrated to us, he said: ‘Amr ibn Yahya informed me, from Muhammad ibn Yahya ibn Habban, from his paternal uncle Wasi’ ibn Habban, that he asked ‘Abdullah ibn ‘Umar about the prayer of the Messenger of Allah (may Allah bless him and grant him peace). He said: "He would say 'Allahu Akbar' (Allah is the Greatest) whenever he raised and whenever he lowered. Then he would say, 'As-salamu ‘alaykum wa rahmatullah' (Peace be upon you and the mercy of Allah) to his right, and 'As-salamu ‘alaykum wa rahmatullah' (Peace be upon you and the mercy of Allah) to his left."