Musnad Ahmad 3:78

'Affan narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, from 'Uthman ibn Hakim, Muhammad ibn al-Munkadir narrated to us, from Humran, from 'Uthman ibn 'Affan, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs wudu and perfects the wudu, his sins exit from his body, until they exit from beneath his fingernails."