Sahih Al Bukhari Hadith Number 747

Narrated Al-Bara:(And Al-Bara was not a liar) Whenever we offered prayer with the Prophet (peace be upon him) and he raised his head from the bowing, we used to remain standing till we saw him prostrating .

10  Call to Prayers (Adhaan)
Hadith number 747
Volume no 1
Chapter: 91 (To cast a look at the Imam during As-Salat (the prayer)