Musannaf Ibn Abi Shayba 33:1322

Abu Usama narrated to us, from Yahya ibn al-Muhallab, from Khalid ibn Salih, from Mu'awiyah ibn Qurrah, who said: Abu al-Darda' said: "Indeed, the heart becomes rusty just as iron becomes rusty." It was said: "And what is its polishing?" He said: "Remembering Allah."