Purity: Onset and Continuity of Bleeding in Ḥayḍ
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 440:
At the beginning of ḥayḍ, it is necessary for the blood to come out. However, it is not necessary for the blood to come out on all three days, and it is sufficient if the blood remains inside the vagina. In the event that during the three days a woman’s bleeding stops for a short time in a manner that is common among all or some women, it is still counted as ḥayḍ.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)