Musnad Ahmad 32:57

Muhammad ibn Fudayl narrated to us, al-Mukhtar ibn Fulful narrated to us, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said one day, having turned away from the prayer, he faced us and said: "O people, I am your Imam, so do not precede me in bowing, nor in prostration, nor in standing, nor in sitting, nor in departing. For indeed, I see you from in front of me and from behind me." And by the One in Whose hand is my soul, if you saw what I saw, you would laugh little and weep much. They said: "O Messenger of Allah, and what did you see?" He said: "I saw Paradise and the Fire."