Sahih Ibn Khuzaymah

Hazrat Hattan bin Abdullah Raqashi narrates that Hazrat Abu Musa Ash'ari (may Allah be pleased with him) led us in prayer. When he sat at the end of his prayer, a person said that prayer has been joined with charity (zakat) (they are mentioned together in the Holy Qur'an). Then, when Hazrat Abu Musa Ash'ari (may Allah be pleased with him) finished the prayer, he asked, "Who among you said these words? Do you not know which words you are to say in your prayer? Indeed, the Messenger of Allah (peace and blessings be upon him) delivered a sermon to us, explained to us the Sunnahs, and taught us our prayer." He (peace and blessings be upon him) said: "When you pray, straighten and align your rows, and one person from among you should lead the prayer. Then, when the imam says the takbir, you should also say the takbir, and when he recites « غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ ‎», you should say 'Ameen', Allah will accept your supplication. And when the imam says the takbir and bows (for ruku'), you should also say the takbir and bow. For the imam bows before you and raises his head from ruku' before you. Therefore, the Prophet (peace and blessings be upon him) said: 'This will be equal (your delay in going into ruku' and rising after him).' When he says the takbir and prostrates, you should also prostrate, for surely the imam prostrates before you and raises his head from prostration before you." In the narration of Bandaar, there is an addition: "So the Prophet (peace and blessings be upon him) said: This will be in exchange for that." Imam Abu Bakr (may Allah have mercy on him) says that what is meant is that the imam bows before you, takes precedence over you, and you raise your head from ruku' after he has raised his head, thus you remain in ruku' (for a while) after him. In this way, remaining in ruku' after the imam has raised his head from ruku' will be in exchange for the precedence the imam takes over the followers in ruku' and prostration.