Surah Al-Baqarah — Ayah 16 (2:16)
أُوْلَٰٓئِكَ ٱلَّذِينَ ٱشۡتَرَوُاْ ٱلضَّلَٰلَةَ بِٱلۡهُدَىٰ فَمَا رَبِحَت تِّجَٰرَتُهُمۡ وَمَا كَانُواْ مُهۡتَدِينَ ١٦
English translation (Saheeh International)
Those are the ones who have purchased error [in exchange] for guidance, so their transaction has brought no profit, nor were they guided.
Brief commentary
It is they who are the fools, because they exchanged faith for disbelief; and this exchange did not help them in any way, because they lost their faith in Allah and lost the way to the truth.