Musnad Abu Ya'la al-Mawsili 131:11

Muhammad ibn Abdullah ibn Numayr narrated to us, Ibn Fudayl narrated to us, Mukhtar ibn Fulful narrated to us, from Anas ibn Malik, who said: The Messenger of Allah (ﷺ) turned away from the prayer one day and said: "By the One in Whose hand is my soul, if you saw what I saw, you would have laughed little and wept much." We said: "O Messenger of Allah, and what did you see?" He said: "I saw Paradise and Hell."