Hadith 864

ثنا عَبْدُ اللَّهِ بْنُ مُحَمَّدِ بْنِ عَبْدِ الْعَزِيزِ ، قَالَ : سُئِلَ أَحْمَدُ بْنُ حَنْبَلٍ ، وَأَنَا أَسْمَعُ عَنِ النُّفَسَاءِ ، كَمْ تَقْعُدُ إِذَا رَأَتِ الدَّمَ ؟ قَالَ " أَرْبَعِينَ يَوْمًا ثُمَّ تَغْتَسِلُ ".
Abdullah bin Muhammad bin Abdul Aziz narrates: Imam Ahmad bin Hanbal was asked, I was hearing this, he was asked about women in nifas (postnatal bleeding): If she sees blood, how long should she remain seated (i.e., refrain from prayer, etc.)? So he replied: "For forty days, after that she will perform ghusl (ritual bath)."
Hadith Reference سنن الدارقطني /  كتاب الحيض / 864
Hadith Takhrij «أخرجه البيهقي فى((سننه الكبير)) برقم: 1641، والدارقطني فى ((سننه)) برقم: 864»