Musannaf Ibn Abi Shaybah

Hazrat Abdullah bin Amr (may Allah be pleased with him) bin Hind al-Jamali (may Allah have mercy on him) narrates that Hazrat Ali (may Allah be pleased with him) said: The beginning of faith is a white spot in the heart. As faith increases, the whiteness also increases, until the entire heart of the believer becomes white with the light of faith. And the beginning of hypocrisy is a black spot in the heart. As hypocrisy increases, the blackness also increases, until the entire heart becomes black with the darkness of hypocrisy. And by the One in Whose hand is my soul, if you were to cut open the heart of a believer, you would surely find it white, and if you were to cut open the heart of a hypocrite, you would surely find it black.