Translation
(1) Purification (Kitab Al-Taharah)
كتاب الطهارة
(118) Chapter: Those Who Said: She Should Perform Ghusl Between The Days (Of Her Menses)
(117) باب مَنْ قَالَ تَغْتَسِلُ بَيْنَ الأَيَّامِ
Sunan Abi Dawud 303 
Muhammad b. 'Uthman asked al-Qasim b. Muhammad about the woman who has a prolonged flow of blood. He replied:
She should abandon prayer during her menstrual period, then wash and pray ; then she should wash during her menstrual period.
حَدَّثَنَا الْقَعْنَبِيُّ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، - يَعْنِي ابْنَ مُحَمَّدٍ - عَنْ مُحَمَّدِ بْنِ عُثْمَانَ، أَنَّهُ سَأَلَ الْقَاسِمَ بْنَ مُحَمَّدٍ عَنِ الْمُسْتَحَاضَةِ، فَقَالَ تَدَعُ الصَّلاَةَ أَيَّامَ أَقْرَائِهَا ثُمَّ تَغْتَسِلُ فَتُصَلِّي ثُمَّ تَغْتَسِلُ فِي الأَيَّامِ ‏.‏
Grade :  Sahih (Al-Albani) صحيح  (الألباني)حكم :
Reference : Sunan Abi Dawud 303
In-book reference : Book 1, Hadith 303
English translation : Book 1, Hadith 303