Musnad Ahmad 1017:94

'Abd al-Rahman narrated to us, Sufyan narrated to us, from 'Alqama ibn Marthad, from Sulayman ibn Burayda, from his father, who said: The Prophet, peace and blessings of Allah be upon him, used to perform ablution for every prayer. When the day of the Conquest came, he performed ablution and wiped over his leather socks, and he prayed the prayers with a single ablution. 'Umar said to him: "O Messenger of Allah, you have done something you did not use to do." He said: "I did it intentionally, O 'Umar."