Musnad Ahmad 937:76

Abu Kamil narrated to us, Hammad ibn Salamah narrated to us, from al-Azraq ibn Qays, from a man from Banu Tamim, who said: We were at the door of Mu'awiyah ibn Abi Sufyan, and among us was Abu Dharr. He said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Fasting the month of patience [Ramadan] and three days of every month is the fasting of a lifetime, and it removes the *maghallat* of the chest." He said: I said: "And what is the *maghallat* of the chest?" He said: "The filth of Satan."