It is narrated from Hazrat Abu Saeed Khudri (may Allah be pleased with him) that once, while praying behind the Prophet (peace be upon him), a man bowed (performed ruku) before the Prophet (peace be upon him) did, and raised his head before the Prophet (peace be upon him) raised his head. After completing the prayer, the Prophet (peace be upon him) asked, "Who did this?" That person presented himself and said, "O Messenger of Allah (peace be upon him)! I did this. Actually, I wanted to know whether you notice it or not." The Prophet (peace be upon him) said, "Avoid making the prayer incomplete. When the Imam bows, you should bow, and when he raises his head, you should raise your head."