Musnad Abu Ya'la al-Mawsili 109:298

Ibn Numayr narrated to us, my father narrated to us, from al-A‘mash, from Abu Sufyan, from Jabir, who said: The Prophet (may Allah bless him and grant him peace) fell from a horse, and his leg was injured. We entered upon him while he was praying, and he gestured to us with his hand. Then we entered the next day while he was praying the obligatory prayer sitting down, and he gestured to us with his hand to sit. Then he finished and said: "When the Imam prays sitting, then pray sitting, and when he prays standing, then pray standing."