Reference: Fataawa as-Siyaam libni Jibreen p131 no.156
Question: If a woman purifies herself [after having completed her menses] in Ramadhaan before the adhaan of the Fajr prayer, is it obligatory upon her to fast [that day]?
Response: If the woman’s [flow of] blood ceases (menses) in the last part of the night in Ramadhaan, then she should take the suhoor until the morning is established (Fajr), and that is because in this state she is pure and fasting is prescribed for her. And it is not correct for her to pray [Fajr] until she makes ghusl, and likewise it is not correct for her to have intercourse [with her husband] until she makes ghusl, as Allaah سبحانه و تعالى said:
{And when they have purified themselves, then go in unto them as Allaah has ordained for you…} al-Baqarah 2:222
