Question:

What is the ruling on delaying (following) the Imaam whether it was during Rukoo (bowing) or whilst in the state of prostration or other than that for the purpose of supplicating , and what is the proof?

Answer:

The Messenger (sallallaahu alaihi wa sallam) said: “The Imaam is to be followed”1 so there is no delaying (of following) the Imaam and there is no preceding him. The Imaam is followed and this is that which the Prophet (sallallaahu alaihi wa sallam) commanded. ” Bow when he bows, prostrate when he prostrates, and when he says ‘Sami Allahu liman-hamidah (Allah heard those who sent praises to Him) say then ‘Rabbana wa lakal-hamd’ (O our Lord! All the praises are for you)”2

So the Imaam is to be followed; this is that which the Messenger  (sallallaahu alaihi wa sallam) commanded with.

1&2 Bukhaari on authority of Anas ibn Malik

http://www.alfawzan.af.org.sa/node/13804