Would a woman be sinful if she made a mistake in determining the time of her purity—due to the absence of the white discharge (al-qassah al-bayda’) and the variation in her menstrual cycle days—either by assuming purity, then performing ghusl and praying, only for some blood to appear later, or, conversely, by assuming the presence of blood, thereby missing the prayer time?

The sign of purity varies among women, and it is often indicated by the emission of white discharge (al-qassah al-bayda') or the cessation of blood. It is not permissible for a woman to rush to perform ghusl until she sees the sign of purity. 'Aisha, may Allah be pleased with her, used to say to women: "Do not rush until you see the white discharge." If a woman makes a mistake in determining the time of purity based on her ijtihad (personal reasoning), there is no sin upon her. However, if she fasts or prays assuming she has become pure, and then it becomes clear otherwise, she must refrain from fasting and praying until she becomes pure. She must make up the fasts she observed, but she does not have to make up the prayers. If she abstains from prayer, assuming she has not become pure, and then it becomes clear that she was pure, she must make up those prayers. Brownish or yellowish discharge before the usual menstruation is not considered menstruation, and if it appears without the signs of menstruation, the woman must make up the prayers she missed.