Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: Sayyidah Sawdah (may Allah be pleased with her) feared that the Messenger of Allah (peace and blessings be upon him) might divorce her, so she said: O Messenger of Allah (peace and blessings be upon him)! Do not divorce me; keep me with you and assign my day to Sayyidah Aishah (may Allah be pleased with her). So he (peace and blessings be upon him) did so, and this verse was revealed: "And if a woman fears from her husband ill-conduct or desertion" [Surah al-Nisa: 128]. He said: Whatever the husband and wife settle upon is permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسم والنشوز / 14735