Maliks Muwatta Book 29, Hadith Number 54.

Section : Idda of Divorce and Divorce of Menstruating Women.

Yahya related to me from Malik from Ibn Shihab from Urwa ibn az-Zubayr from A’isha, umm al-muminin, that she took Hafsa ibn Abd ar-Rahman ibn Abi Bakr as-Siddiq into her house when she had entered the third period of her idda. Ibn Shihab said, “That was mentioned to Amra bint Abd ar-Rahman, and she said that Urwa had spoken the truth and people had argued with A’isha about it. They said that Allah, the Blessed, the Exalted, said in His Book, ‘Three quru.’ A’isha said, ‘You spoke the truth. Do you know what quru are? Quru are times of becoming pure after menstruation.'”