Muwatta Malik #138
Purity
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
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."
About Muwatta Malik
This hadith is #138 from Muwatta Malik, in the Book of Purity.