Musannaf Ibn Abi Shayba 23:14

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Sulayman ibn Maysarah, from Tariq ibn Shihab, who said: Abdullah said: "Indeed, a man commits a sin, and a black dot is dotted upon his heart. Then he commits another sin, and it is dotted, until his heart becomes the color of a dusty-colored sheep."