Musnad Ahmad 30:102

'Abd al-Rahman narrated to us, Malik narrated to us, from al-Zuhri, from Abu Salamah, that Abu Hurayrah used to say "Allahu Akbar" (God is the Greatest) whenever he lowered and raised [his body], and he would say: "I am the most similar of you in prayer to the Messenger of Allah, may Allah bless him and grant him peace."