Sahih Al Bukhari Hadith Number 734

Narrated Abu Huraira: The Prophet (peace be upon him) said, “The Imam is to be followed. Say the Takbir when he says it; bow if he bows; if he says ‘Sami`a l-lahu liman hamidah’, say, ‘ Rabbana wa laka l-hamd’, prostrate if he prostrates and pray sitting altogether if he prays sitting.”

10  Call to Prayers (Adhaan)
Hadith number 734
Volume no 1
Chapter: 82 (The necessity of saying the Takbir, i.e., Allahu Akbar (Allah is the Most Great) and the commencement of As-Salat (the prayer)