Sahih Muslim Book 4, Hadith Number 956.

Chapter : The duty of the Imam is to be brief and perfect in prayer.

Thabit reported it on the authority of Anas: While leading you in prayer I do not shorten anything in the prayer. I pray as I saw the Messenger of Allah (may peace be upon him) leading us. He (Thabit) said: Anas used to do that which I do not see you doing; when he lifted his head from bowing he stood up (so long) that one would say: He has forgotten (to bow down in prostration). And when he lifted his head from prostration, he stayed in that position, till someone would say: He has forgotten (to bow down in prostration for the second sajda).