It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Hafsah bint Abd al-Rahman bin Abi Bakr al-Siddiq (may Allah be pleased with them both) moved (out of her waiting period) when she entered the blood of her third menstruation. Ibn Shihab said: I mentioned this to Amrah bint Abd al-Rahman, and she said: Urwah has spoken the truth. Some people argued with her (Aishah) about this and said: Allah Almighty has said: "three quru'" [Surah al-Baqarah: 228]. Sayyidah Aishah (may Allah be pleased with her) said: Do you know what al-aqra' are? Al-aqra' means the periods of purity.
Ibn Shihab said: I heard Abu Bakr bin Abd al-Rahman say: I have not met any of our jurists except that he says this, meaning what Aishah (may Allah be pleased with her) said.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15382