أَلَمْ تَرَ إِلَى ٱلَّذِينَ يُزَكُّونَ أَنفُسَهُم ۚ بَلِ ٱللَّهُ يُزَكِّى مَن يَشَآءُ وَلَا يُظْلَمُونَ فَتِيلًا
Alam tara ilal lazeena yuzakkoona anfusahum; balil laahu yuzakkee mai yashaaa'u wa laa yuzlamoona fateelaa
Have you not seen those who claim purity for themselves? Rather, God purifies whom he wills, and they will not be wronged by so much as the thread of a date stone.
Purification here is God's own act: tazkiya belongs to him, so what a person claims about their own standing adds nothing to it. The word fatīl names the fibre running along the groove of a date stone, proverbial in Arabic for the smallest imaginable amount, and it arrives as reassurance rather than threat: whatever a person has earned is credited down to the last thread.