﴿ أَلَمْ تَرَ إِلَى ٱلَّذِينَ يُزَكُّونَ أَنفُسَهُم ۚ بَلِ ٱللَّهُ يُزَكِّى مَن يَشَآءُ وَلَا يُظْلَمُونَ فَتِيلًا ﴾

“Art thou not aware of those who consider themselves pure? Nay, but it is God who causes whomever He wills to grow in purity; and none shall be wronged by as much as a hair's breadth.”

(Hast thou not seen) have you not been informed in the Scripture about (those who praise themselves for purity?) those who absolve themselves from transgression. The reference here is to Buhayr Ibn 'Amr and Marhab Ibn Zayd. (Nay, Allah purifieth) absolves from sins (whom He will) whoever is fit for it, (and they will not be wronged even the hair upon a date stone) their sins will not be diminished even by the hair upon a date stone.