Musnad Ahmad 2:261

'Ali ibn 'Asim narrated to us, Yazid ibn Abi Ziyad informed us, from 'Asim ibn 'Ubayd Allah, from his father or his grandfather—the doubt is from Yazid—from 'Umar, who said: I saw the Messenger of Allah (may Allah bless him and grant him peace) perform ablution after passing wind, and he wiped over his leather socks and prayed.