Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 768
Abu Salama reported that Abu Huraira recited takbir in prayer on all occasions of rising and kneeling. We said: O Abu Huraira, what is this takbir? He said: Verily it is the prayer of the Messenger of Allah (may peace be upon him).