The Quran

الَّذينَ يُظاهِرونَ مِنكُم مِن نِسائِهِم ما هُنَّ أُمَّهاتِهِم ۖ إِن أُمَّهاتُهُم إِلَّا اللّائي وَلَدنَهُم ۚ وَإِنَّهُم لَيَقولونَ مُنكَرًا مِنَ القَولِ وَزورًا ۚ وَإِنَّ اللَّهَ لَعَفُوٌّ غَفورٌ

(Saheeh International)

Those who pronounce thihār¹ among you [to separate] from their wives – they are not [consequently] their mothers. Their mothers are none but those who gave birth to them. And indeed, they are saying an objectionable statement and a falsehood. But indeed, Allāh is Pardoning and Forgiving.

The saying by a husband to his wife, "You are to me like the back of my mother," meaning unlawful to approach. This was a type of divorce practiced by the Arabs before the prophethood of Muḥammad (ṣ).