Musnad Ahmad 3:118

Yunus narrated to us, Layth narrated to us, from Yazid ibn Abi Habib, from Abdullah—meaning Ibn Abi Salamah—and Nafi‘ ibn Jubayr ibn Mut‘im, from Mu‘adh ibn Abd al-Rahman al-Taymi, from Humran, the freed slave of Uthman, from Uthman ibn Affan, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Whoever performs wudu and perfects the wudu, then walks to an obligatory prayer and prays it, his sin will be forgiven."