Musannaf Ibn Abi Shayba 3:3607

Yazid ibn Harun narrated to us, he said: Al-Mas'udi informed us, from al-Qasim, who said: When Mu'adh was near death, he said: "There is no one who prays four units of voluntary prayer after an obligatory prayer, but that any sin he committed that day is wiped away from him, except for associating partners with Allah, until the sun sets."