Muwatta Malik 1217

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ الْفُضَيْلِ بْنِ أَبِي عَبْدِ اللَّهِ، مَوْلَى الْمَهْرِيِّ أَنَّ الْقَاسِمَ بْنَ مُحَمَّدٍ، وَسَالِمَ بْنَ عَبْدِ اللَّهِ، كَانَا يَقُولاَنِ إِذَا طُلِّقَتِ الْمَرْأَةُ فَدَخَلَتْ فِي الدَّمِ مِنَ الْحَيْضَةِ الثَّالِثَةِ فَقَدْ بَانَتْ مِنْهُ وَحَلَّتْ ‏.‏

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

Reference

Muwatta Malik 1217

In-book

Book 29, Hadith 14