Musnad Ahmad 27:1937

Rawh narrated to us, Ibn Jurayj narrated to us, ‘Amr ibn Yahya informed me, from Muhammad ibn Yahya ibn Habban, from his paternal uncle Wasi’, that he asked ‘Abdullah ibn ‘Umar about the prayer of the Messenger of Allah (may Allah bless him and grant him peace). He said: "Allah is the Greatest," every time he lowered [himself] and every time he raised [himself]. Then he would say: "Peace be upon you and the mercy of Allah," to his right, [and] "Peace be upon you and the mercy of Allah," to his left.