Sahih Muslim Book 4, Hadith Number 857.

Chapter : Command to observe prayer well, perfecting it, and devotion in it.

Anas reported: The Messenger of Allah (may peace be upon him) one day led us in the prayer and when he completed the Prayer he turned his face towards us and said: O People, I am your Imam, so do not precede me in bowing and prostration and in standing and turning (faces, i.e. In pronouncing salutation), for I see you in front of me and behind me, and then said: By Him in Whose hand Is the life of Muhammad, if you could see what I see, you would have laughed little and wept much more. They said: What did you see, Messenger of Allah? He replied: (I saw) Paradise and Hell.