Musnad Ahmad 626:39

Muhammad ibn Abi ‘Adi narrated to us, from Humayd, from Bakr, from Hamzah ibn al-Mughirah ibn Shu‘bah, from his father, who said: The Messenger of Allah (ﷺ) lagged behind, so he relieved himself. He said, "Do you have purification water with you?" He said: So I followed him with a vessel containing water. He washed his hands and his face. Then he went to uncover his forearms, and there was tightness in the sleeves of his cloak, so he brought his hands out from under the cloak and washed his forearms. Then he wiped over his turban and his leather socks. He mounted, and I mounted my riding beast, and we reached the people while ‘Abd al-Rahman ibn ‘Awf had already prayed one rak‘ah with them. When he sensed the Prophet (ﷺ), he went to step back, but he gestured to him to complete the prayer and said, "You have done well; do likewise."