It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both): If he divorces her while she is menstruating, she does not count that menstruation (in her waiting period). Yahya (may Allah have mercy on him) said: This is gharib (unusual); no one narrates it except Abd al-Wahhab al-Thaqafi. Imam al-Bayhaqi (may Allah have mercy on him) said: Yahya bin Ayyub al-Misri narrated its meaning from Ubaydullah, and it has been narrated to us from Sayyiduna Zayd bin Thabit (may Allah be pleased with him) that he said: When a man divorces his wife while she is in postnatal bleeding (nifas), the blood of nifas is not counted in her waiting period.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15402
It is narrated from the jurists of the people of Madinah that whoever divorces his wife while she is menstruating or in postnatal bleeding (nifas), her waiting period is three menstruations, apart from the bleeding in which she was divorced.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15403