Sayyiduna Uthman bin Abi al-As (may Allah be pleased with him) says: A period of forty days has been fixed for women in postnatal bleeding (nifas).
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Isma'il bin Muslim al-Makki, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1546