It is narrated from al-Fudayl bin Abi Abdullah, the freed client of al-Mahri, that he asked al-Qasim bin Muhammad (may Allah have mercy on him) and Salim bin Abdullah (may Allah have mercy on him): When a woman is divorced and she enters the blood of her third menstruation, what is the ruling? They both said: She has become irrevocably separated (ba'in) and has become lawful (to marry), that is, her waiting period has ended.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15390