Sayyiduna Jabir (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) fell from his horse onto the trunk of a date palm, and his ankle was dislocated, so he (peace and blessings be upon him) stayed in the house of Sayyidah Aishah (may Allah be pleased with her). We came to visit him (peace and blessings be upon him) in his illness and found him (peace and blessings be upon him) offering a voluntary prayer sitting, while we were standing. Then we came again, and the Prophet (peace and blessings be upon him) was offering an obligatory prayer sitting. We stood behind him (peace and blessings be upon him), and he (peace and blessings be upon him) gestured to us, so we sat down. Then he said: "Follow your imam. If he prays sitting, you too pray sitting, and if he prays standing, you too pray standing; and do not do as the Persians do for their great men."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5074