Qur'an 24:6

وَٱلَّذِينَ يَرْمُونَ أَزْوَٰجَهُمْ وَلَمْ يَكُن لَّهُمْ شُهَدَآءُ إِلَّآ أَنفُسُهُمْ فَشَهَـٰدَةُ أَحَدِهِمْ أَرْبَعُ شَهَـٰدَٰتٍۭ بِٱللَّهِ ۙ إِنَّهُۥ لَمِنَ ٱلصَّـٰدِقِينَ

As for those who accuse their wives but have no witnesses except themselves; let the testimony of one of them be four testimonies, (swearing) by Allah that he is of those who speak the truth;