Sunan An-Nasai, Chapter: The Book of Purification

"There are two things which I never asked anyone about after I saw the Messenger of Allah (saw). He was with us on a journey and he went away to relieve himself, then he came and performed Wudu, and he wiped over his forehead and two sides of his Imamah, and he wiped over his Khuffs." He said: "And (the other issue) the Imam's Salah behind one of his followers. I saw the Messenger of Allah (saw) when he was on a journey and time for prayer came. The Prophet (saw) could not join them, so they called Iqamah and they asked Ibn 'Awf to lead them in prayer. Then the Messenger of Allah (saw) came and offered the remainder of the prayer behind Ibn Awf, then when Ibn 'Awf said the Salah, the Prophet (saw) stood up and completed what he had missed (of the prayer)." (Sahih)