Musnad Ahmad 5:324

Ali ibn Bahr narrated to us, Baqiyyah ibn al-Walid al-Homsi narrated to us, al-Wadin ibn 'Ata' narrated to me, from Mahfuz ibn 'Alqamah, from 'Abd al-Rahman ibn 'A'idh al-Azdi, from Ali ibn Abi Talib, from the Prophet (peace be upon him), who said: "The anus is the tie of the eye; so whoever sleeps, let him perform ablution."