Purity: Ḥayḍ During Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 454:
If a woman becomes ḥāʾiḍ during prayers, her prayers are invalid; and based on obligatory precaution, this applies even if ḥayḍ occurs after the last sajdah and before the last word of the salutation (salām) of the prayer.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)