Musnad Ahmad 3:85

Hajjaj and Yunus narrated to us, they said: Layth narrated to us. Hajjaj said: Yazid ibn Abi Habib narrated to me, from Abdullah 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, that he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Whoever performs ablution and perfects the ablution, then walks to an obligatory prayer and prays it, his sin will be forgiven for him."