Musnad Ahmad 626:96

Sulayman ibn Dawud al-Hashimi narrated to us, Isma'il—meaning Ibn Ja'far—narrated to us, Sharik—meaning Ibn 'Abd Allah ibn Abi Namir—informed me that he heard Abu al-Sa'ib, the freed slave of Hisham ibn Zuhrah, saying: I heard al-Mughirah ibn Shu'bah saying: "The Prophet, may Allah bless him and grant him peace, went out on a journey and stopped at a place. The Prophet, may Allah bless him and grant him peace, went to relieve himself, so I followed him with a water skin. I poured [water] for him, and he performed ablution and wiped over his leather socks."