Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) said concerning this saying of Allah Almighty, "And if a woman fears from her husband ill-conduct or desertion, there is no blame on them if they make terms of settlement" [Surah al-Nisa: 128]: (It is) a man who has two wives, one of whom has grown old or is unattractive, and he intends to divorce her. So the woman makes a settlement that he will stay with her one night and with the other several nights, and that he will not separate from her. Whatever she is content with, there is no harm in it; and if she goes back on it, he must divide the turns equally between them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسم والنشوز / 14737