Chapter on the Rules of Menstruation, Istihadah, and Nifas, and Purification From Them

Ali ibn Mahziyar narrated from Al-Husayn ibn Sa'id, from Zur'ah, from Sama'ah, who said: I asked Imam عليه السلام about a woman who sees blood before the time of her usual menstruation. Imam عليه السلام said: 'She should refrain from praying because sometimes her period may come earlier than usual. If the bleeding exceeds the days of her usual menstruation, she should wait for three days after her usual period has passed. If, after waiting for three days, the bleeding does not stop, she should follow the rulings applicable to a mustahadha (a woman experiencing irregular bleeding).'"