The Prophet (peace and blessings be upon him) said:
"A woman with istihadah (irregular bleeding) should take a ritual bath once from one menstrual period to the next."
Hadith Referenceصحیح الجامع الصغیر / حرف الميم / 6699
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[طس] عن ابن عمرو. الروض النضير ٥٩٠: طص.