It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The time set for women in nifas is forty nights, unless she sees purity before that."
Zayd al-Ammi, Sallam bin Salm al-Mada'ini, al-Arzami and al-Ala' bin Kathir al-Dimashqi are all da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1619