Musannaf Ibn Abi Shayba 23:13

Abu Usama narrated to us, he said: Awf narrated to us, from Abdullah ibn Amr ibn Hind al-Jamali, he said: Ali said: Faith appears as a white dot in the heart. Whenever faith increases, it increases in whiteness until the entire heart becomes white. And hypocrisy appears as a black dot in the heart. Whenever hypocrisy increases, it increases in blackness until the entire heart becomes black. By the One in whose hand is my soul, if you were to split open the heart of a believer, you would find it white; and if you were to split open the heart of a hypocrite, you would find it black.