Hisham bin Urwah (may Allah have mercy on him) reports from his father (Urwah, may Allah have mercy on him) that he said: There is nothing upon the woman with istihadah (irregular bleeding) except a single ritual bath (ghusl); then after that she should perform ablution for the prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1644