Amr bin Wahb al-Thaqafi (may Allah have mercy on him) narrates that Mughirah bin Shu'bah (may Allah be pleased with him) said: There are two matters about which I do not ask anyone after having witnessed them with the Messenger of Allah (peace and blessings be upon him): wiping over the leather socks, and the imam (ruler) praying behind one of his subjects. We were with the Messenger of Allah (peace and blessings be upon him) on a journey, and we stopped at a place, and the Prophet (peace and blessings be upon him) went off to relieve himself. When he (peace and blessings be upon him) had finished, I brought him (peace and blessings be upon him) water, and he (peace and blessings be upon him) performed ablution - he (peace and blessings be upon him) was wearing a cloak with tight sleeves - so he (peace and blessings be upon him) brought his hand out from beneath the cloak and wiped over his forelock, his turban and his leather socks. Then we came to the people while they were praying, and the people had put Abd al-Rahman bin Awf (may Allah be pleased with him) forward (to lead), and we caught one rak'ah with them; then the Messenger of Allah (peace and blessings be upon him) completed the remaining rak'ah.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18155