The Quran Asks 4-49-50

Did you not consider those who attribute purity to them selves? No, Allah purifies whom He wills and they are not dealt with unjustly even to the extend of the hair on a datestone, see how they fabricate falsehood about Allah, and that itself is sufficient as a clear sin (to prove that they are not pure). (Al Quran, Chapter 4: Verses 49-50)