Musannaf Ibn Abi Shayba 1:49

'Abdah ibn Sulayman narrated to us, from 'Uthman ibn Hakim, from Muhammad ibn al-Munkadir, from Humran, who said: I heard 'Uthman say: "Whoever performs wudu, and performs the wudu well, and makes it thorough, and completes it, his sins exit from his body until they exit from beneath his fingernails."