Musnad Ahmad 1665

It was narrated from "Abdullah bin al-Waleed that he heard Abu Salamah bin ‘AbdurRahman narrate from his father that he was with the Prophet (ﷺ) on a journey. The Prophet (ﷺ) went to relieve himself, then the time for prayer came and they got up to pray, and asked ‘Abdur Rahman to lead them in prayer. Then the Prophet (ﷺ) came and prayed one rak"ah with the people behind him. When he said the salam, he said: "You did the right thing" or "You did well.”