An-Nisa' - 4:49

أَلَمْ تَرَ إِلَى الَّذِينَ يُزَكُّونَ أَنفُسَهُمْ بَلِ اللّهُ يُزَكِّي مَن يَشَاء وَلاَ يُظْلَمُونَ فَتِيلاً
[4:49]
Have you not considered those who attribute purity to themselves? Nay, Allah purifies whom He pleases; and they shall not be wronged the husk of a date stone.
Hast thou not turned Thy vision to those who claim sanctity for themselves? Nay-but Allah Doth sanctify whom He pleaseth. But never will they fail to receive justice in the least little thing.
Hast thou not seen those who praise themselves for purity? Nay, Allah purifieth whom He will, and they will not be wronged even the hair upon a date-stone.