It is narrated from Sayyidah Rubayyi' bint Mu'awwidh (may Allah be pleased with her) that she obtained khul' in the blessed time of the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) ordered her, or she was ordered, "to observe an 'iddah of one menstrual period."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15599