From Sahih Muslim (Hadith 921):
Anas b. Malik reported:The Apostle of Allah (ﷺ) fell down from a horse and his right side was grazed. We went to him to inquire after his health when the time of prayer came. He led us in prayer in a sitting posture and we said prayer behind him sitting, and when he finished the prayer hesaid: The Imam is appointed only to be followed; so when he recites takbir, you should also recite that; when he prostrates, you should also prostrate; when he rises up, you should also rise up, and when he said" God listens to him who praises Him," you should say:" Our Lord, to Thee be the praise," and when he prays sitting, all of you should pray sitting
Reference: Muslim 921