It is narrated from Aisha (may Allah be pleased with her) that she said: "When a divorced woman begins to experience the blood of her third menstruation, she has completed her waiting period ('iddah) from her husband."
Hadith Referenceمسند الإمام الشافعي / كتاب العدد والسكنى والنفقات / 1290
Hadith Takhrijاسناده صحیح اخرجه البيهقى فى المعرفة السنن والآثار (4606)۔