The Messenger of Allah ﷺ had stayed behind, and when we reached the people, Abdul-Rahman bin Awf had already led them in one rakah of the prayer. When he sensed the Prophet's ﷺ presence, he began to step back, but the Prophet ﷺ gestured to him to complete the prayer.Afterwards he said, "You have done well. Continue doing so."
حدثنا محمد بن المثنى حدثنا ابن أبي عدي عن حميد عن بكر بن عبد الله
تخلف رسول الله ﷺ فانتهينا إلى القوم وقد صلى بهم عبد الرحمن بن عوف ركعة فلما أحس بالنبي ﷺ ذهب يتأخر فأومأ إليه النبي ﷺ أن يتم الصلاة قال وقد أحسنت كذلك فافعل
Narrated by Muslim, in the Book of Purification, chapter on wiping over the forelock and turban, after hadith no. 274, from Humayd al-Tawil, with similar wording. He also has another route for it in the Book of Prayer, chapter on wiping over the socks and other similar matters.