(a) It is narrated from Hittan bin Abdullah al-Raqashi that Abu Musa al-Ash'ari (may Allah be pleased with him) led the people in prayer, and he then mentioned the complete hadith. In it there is also that Abu Musa al-Ash'ari (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) addressed us, taught us the prayer and explained its method to us. He (peace and blessings be upon him) said: "When you stand to pray, straighten your rows. When the imam says the takbir, say the takbir; and when he says 'Not of those who have earned anger, nor of those who go astray' [Surah al-Fatihah: 7], say 'Amin'; Allah will answer your supplication. And when he says the takbir and bows, then bow, for the imam says the takbir before you and will also raise his head from bowing before you." And the Prophet (peace and blessings be upon him) said: "The follower should keep pace with him. And when the imam says 'Sami'allahu liman hamidah' (Allah has heard the one who praised Him), say 'Rabbana lakal-hamd' (Our Lord, to You belongs all praise); Allah will answer your supplication."
(b) It is narrated from Zahdam al-Jarmi: I prayed behind Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) and he said to us: When the imam says 'Allahu akbar', then you too, following him, say 'Allahu akbar'; and when he says 'Sami'allahu liman hamidah' while raising his head from bowing, then you too say the like of it.
The authentic narration is the first one.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2618