Musnad Ahmad 32:336

'Abd al-Samad ibn 'Abd al-Warith narrated to us, Za'idah 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: "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: "What did you see?" He said: "I saw Paradise and Hell." He urged them to pray, and forbade them from preceding him when their Imam is in bowing (ruku') and prostration (sujud), and from departing before he departs from the prayer. He said to them: "I see you from in front of me and from behind me." I asked Anas about the prayer of the sick person, and he said: "He bows and prostrates while sitting in the obligatory prayer."