Musnad Ahmad 32:627

Aswad ibn Amir narrated to us, Zuhayr narrated to us, from al-Mukhtar ibn Fulful, that Anas ibn Malik narrated to them, saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I am an Imam for you, so do not precede me in bowing, nor in prostration, nor in standing, for I see you from in front of me and from behind me." And by the One in Whose hand is the soul of Muhammad, if you saw what I saw, you would laugh little and weep much. They said: "O Messenger of Allah, what did you see?" He said: "I saw Paradise and the Fire."