From Sahih Muslim (Hadith 759):
On the authority of 'A'isha:Umm Habiba asked the Messenger of Allah (ﷺ) about the blood (which flows beyond the period of menstruation). 'A'isha said: I saw her wash-tub full of blood. The Messenger of Allah (ﷺ) said: Remain away (from prayer) equal (to the length of time) that your menses prevented you. After this (after the period of usual courses) bathe yourself and offer prayer
Reference: Muslim 759