Musnad Abu Ya'la al-Mawsili 155:152

Ahmad ibn Ibrahim al-Dawraqi narrated to us, saying: Mubashir ibn Isma'il al-Halabi narrated to us, from al-Awza'i, from Yahya ibn Abi Kathir, from Abu Salamah, who said: I saw Abu Hurayrah performing this takbir (saying Allahu Akbar) which you see. So I said to him, "O Abu Hurayrah, what is this takbir?" He said, "It is indeed the prayer of the Messenger of Allah, may Allah bless him and grant him peace."