The Messenger of Allah ﷺ said, "By the One in whose hand is the soul of Muhammad, if you saw what I have seen, you would laugh little and weep much."They asked, "What did you see?"He replied, "I saw Paradise and the Fire."He urged them to be diligent in prayer and forbade them from preceding him in bowing and prostration when he was leading them, or from leaving before he concluded the prayer. He told them, "I see you from in front of me and from behind me."I asked Anas about the prayer of a sick person, and he said, "He bows and prostrates while seated in the obligatory prayer."
حدثنا عبد الصمد بن عبد الوارث حدثنا زائدة حدثنا المختار بن فلفل
قال رسول الله ﷺ والذي نفس محمد بيده لو رأيتم ما رأيت لضحكتم قليلا ولبكيتم كثيرا قالوا ما رأيت قال رأيت الجنة والنار وحضهم على الصلاة ونهاهم أن يسبقوه إذا كان إمامهم في الركوع والسجود وأن ينصرفوا قبل انصرافه من الصلاة وقال لهم إني أراكم من أمامي ومن خلفي وسألت أنسا عن صلاة المريض فقال يركع ويسجد قاعدا في المكتوبة
Its isnad is sahih. Zaidah is Ibn Qudama, a reliable and firmly trustworthy narrator. The hadith has already been mentioned in similar wording, and it is found in Nasai, 2/83, in the chapter on inattentiveness during prayer, on the prohibition of anticipating the imam.