مُوَطَّأُ مَالِكٍ
Muwatta Malik 138
Purity
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ سُمَىٍّ، مَوْلَى أَبِي بَكْرِ بْنِ عَبْدِ الرَّحْمَنِ أَنَّ الْقَعْقَاعَ بْنَ حَكِيمٍ، وَزَيْدَ بْنَ أَسْلَمَ، أَرْسَلاَهُ إِلَى سَعِيدِ بْنِ الْمُسَيَّبِ يَسْأَلُهُ كَيْفَ تَغْتَسِلُ الْمُسْتَحَاضَةُ فَقَالَ تَغْتَسِلُ مِنْ طُهْرٍ إِلَى طُهْرٍ وَتَتَوَضَّأُ لِكُلِّ صَلاَةٍ فَإِنْ غَلَبَهَا الدَّمُ اسْتَثْفَرَتْ .
Yahya related to me from Malik from Sumayy, the mawla of Abu Bakr ibn Abd ar-Rahman that al-Qaqa ibn Hakim and Zayd ibn Aslam sent him to Said ibn al-Musayyab to ask how a woman who was bleeding as if menstruating should do ghusl. Said said, "She does a ghusl to cover from the end of one period to the end of the next, and does wudu for every prayer, and if bleeding overtakes her she should bind her private parts."
Arabic/English book reference: Book 2, Hadith 109