It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to teach us: "Do not hasten ahead of the imam in bowing. When he says the takbir, say the takbir, and when the imam says: 'Not of those who have earned anger, nor of those who go astray' [Surah al-Fatihah: 7], say Amin; for when your voice coincides with the words of the angels, all the people present in the mosque will be forgiven. When the imam bows, bow, 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), and when the imam prostrates, prostrate."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2593