Musnad Ahmad 3:8

'Abd al-Rahman ibn Mahdi narrated to us, Shu'bah narrated to us, from Jami' ibn Shaddad, who said: I heard Humran ibn Aban narrating from 'Uthman, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs the ablution completely, as Allah the Almighty has commanded him, then the prescribed prayers are expiations for what is between them."