Musnad Ahmad 1017:39

Waki’ narrated to us, Sufyan informed us, from ‘Alqamah ibn Marthad, from Sulayman ibn Buraydah, from his father, that when the Messenger of Allah (peace and blessings of Allah be upon him) was on the day of the Conquest—the conquest of Makkah—he performed ablution and wiped over his leather socks. ‘Umar said to him, "I saw you, O Messenger of Allah, do something today that you were not accustomed to doing." He said, "I did it intentionally, O ‘Umar."