Purity: Internal vs. External Discharge of Istiḥāḍah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 405:
If when the blood of istiḥāḍah is first discharged, it remains inside and does not come out, this does not invalidate a woman’s wuḍūʾ or ghusl; but if it comes out – however little it may be – then this invalidates her wuḍūʾ and ghusl.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)