Purity: Istibra When Nifas Stops
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 506:
If a woman’s nifās stops and she deems it probable that there is blood inside, she must perform ghusl as a precautionary measure and the ritual acts of worship, or she must perform istibrāʾ; it is not permitted for her to refrain from performing the ritual acts of worship without performing istibrāʾ. The method of performing istibrāʾ was mentioned in Ruling 495.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)