Majma al-Zawaid — Hadith 2744

Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "The imam is appointed only so that he may be followed. When he says the takbir, you say the takbir; when he goes into ruku' (bowing), you go into ruku'; when he says 'Sami'a Allahu liman hamidah' (Allah hears the one who praises Him), you say 'Al-hamdu lillah' (All praise is for Allah)."
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat. It is mentioned in the Sahih, but the words "Al-hamdu lillah" are not mentioned there. The narrators of this narration have been declared trustworthy.