Musnad Ahmad 33:1134

Abu Ja'far Muhammad ibn Ja'far al-Mada'ini narrated to us, Warqa' informed us, from Mansur, from Salim ibn Abi al-Ja'd, from Jabir ibn Abdullah, who said: The foot of the Messenger of Allah (peace and blessings of Allah be upon him) was sprained, so we entered upon him. We found him—or he came out to us—sitting in his chamber in front of a room. He prayed sitting, and we stood behind him and prayed. When he finished the prayer, he said: "When I pray sitting, then pray sitting, and when I pray standing, then pray standing. And do not stand as the Persians stand for their tyrants, or for their kings."