Musnad Ahmad 32:1335

'Abd al-Rahman ibn Mahdi narrated to us, Za'idah narrated to us, from al-Mukhtar ibn Fulful, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "By the One in Whose hand is the soul of Muhammad, if you saw what I saw, you would weep much and laugh little." They said: "What did you see, O Messenger of Allah?" He said: "I saw Paradise and the Fire." And he forbade them from preceding him when he led them in bowing (ruku') and prostration (sujud), and from departing before he departed from the prayer. He said: "Indeed, I see you from in front of me and from behind me."