(a) It is narrated from al-Hasan (may Allah have mercy on him) that when women in nifas see (their blood), they should wait for fifty nights.
(b) In this there is evidence that he interpreted what he narrated from Uthman bin Abi al-As (may Allah be pleased with him) about forty days to mean that Uthman bin Abi al-As (may Allah be pleased with him) held that if she becomes pure before forty days, her husband will not have intercourse with her until forty days are complete. And we have narrated from Ibn Abbas (may Allah be pleased with them both) what indicates that he held the opposite view regarding what is less than forty days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1615