Musnad Ahmad 626:87

Waki’ narrated to us, Bakir bin ‘Amir narrated to us, from Ibn Abi Nu’m, from Al-Mughirah bin Shu’bah, who said: I was with the Prophet, peace and blessings of Allah be upon him, on a journey. He relieved himself, then performed ablution (wudu), and wiped over his leather socks. I said: "O Messenger of Allah, you forgot." He said: "Rather, you forgot. With this, my Lord, the Almighty and Majestic, commanded me."