Sunan ad-Darimi

Sayyiduna Abu Musa Ash'ari (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) delivered a sermon to us and taught us our prayer and explained to us our Sunnah. (The narrator said:) I think he (peace and blessings be upon him) said: "When the iqamah is called for prayer, then one of you should lead you in prayer. When he (the imam) says the takbir, then you also say the takbir (Allahu Akbar), and when he recites « غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ », then you say Ameen, Allah the Exalted will accept your supplication. And when he (the imam) says Allahu Akbar and bows (for ruku'), then you also say the takbir and go into ruku' (but keep in mind) the imam will go into ruku' before you and will rise from ruku' before you." The Noble Prophet (peace and blessings be upon him) said: "This is in response to the imam (i.e., takbir, ruku', etc.), and when he (the imam) says «سَمِعَ اللّٰهُ لِمَنْ حَمِدَهُ», then you say «اللّٰهُمَّ رَبَّنَا لَكَ الْحَمْدُ»," or he said: "«رَبَّنَا لَكَ الْحَمْدُ»." Allah the Exalted has said this by the tongue of His Prophet: «سَمِعَ اللّٰهُ لِمَنْ حَمِدَهُ». (That is, you should say: «سَمِعَ اللّٰهُ لِمَنْ حَمِدَهُ» Allah has heard the one who praised Him.)