Maliks Muwatta Book 29, Hadith Number 59.
Section : Idda of Divorce and Divorce of Menstruating Women.
Yahya related to me from Malik from al-Fudayl ibn Abi Abdullah, the mawla of al-Mahri that al-Qasim ibn Muhammad and Salim ibn Abdullah said, “When a woman is divorced and begins her third period, she is clearly separated from him and is free to marry again.”