أَلَم تَرَ إِلَى الَّذينَ يُزَكّونَ أَنفُسَهُم ۚ بَلِ اللَّهُ يُزَكّي مَن يَشاءُ وَلا يُظلَمونَ فَتيلًا

Have you not seen those who claim themselves to be pure? Rather, Allah purifies whom He wills, and injustice is not done to them, [even] as much as a thread [inside a date seed].