It was narrated that Umm Salamah said:"A woman asked the Prophet: 'I bleed continuously and I do not become pure. Should I give up the prayer?' He said: 'No, but leave off praying for the number of days and nights that used to menstruate.'" (One of the narrators) Abu Bakr (Ibn Abu Shaibah) said in this Hadith: "Estimate the number of days in the month, then take a bath and cover your private part with a cloth and perform prayer."
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَعَلِيُّ بْنُ مُحَمَّدٍ، قَالاَ حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ عُبَيْدِ اللَّهِ بْنِ عُمَرَ، عَنْ نَافِعٍ، عَنْ سُلَيْمَانَ بْنِ يَسَارٍ، عَنْ أُمِّ سَلَمَةَ، قَالَتْ سَأَلَتِ امْرَأَةٌ النَّبِيَّ ـ صلى الله عليه وسلم ـ قَالَتْ إِنِّي أُسْتَحَاضُ فَلاَ أَطْهُرُ أَفَأَدَعُ الصَّلاَةَ قَالَ " لاَ وَلَكِنْ دَعِي قَدْرَ الأَيَّامِ وَاللَّيَالِي الَّتِي كُنْتِ تَحِيضِينَ " . قَالَ أَبُو بَكْرٍ فِي حَدِيثِهِ " وَقَدْرَهُنَّ مِنَ الشَّهْرِ ثُمَّ اغْتَسِلِي وَاسْتَثْفِرِي بِثَوْبٍ وَصَلِّي " .
IbnMaja
The Book of Purification and its Sunnah • Hadith 623