Authentic Supplication Number 084.

'Abi Hurayrata  is reported as saying: The Prophet  said: "Verily the leader ('Imam) was put in place in order to be followed, so when he makes takbeer, then, make takbeer; when he bows, then, bow; when he says sami`a 'allahu liman hamidahu then, say: Rabbana wa lakal-hamdu (96); when he prostrates, then, prostrate; and if he prays sitting down, then, pray, sitting down, altogether.

Footnotes.
(96) The wording as reported by Muslim is 'Allahumma rabbana lakal-hamdu.

Reporters.
'Al- Adawee said it was sound, and that it was reported by Al-Bukhari, page 216 of volume 2 of Fath Al-Baree; Muslim, page 404 of volume 1; Ibn Majah #1239; and An-Nisa'ee with a different wording, page 142 of volume 2.