Allah does not accept the prayer of any woman who has reached the age of menstruation unless [she is] veiled
عن عائشة رضي الله عنها عن النبي صلى الله عليه وسلم أنه قال: «لا يَقْبَل الله صلاة حَائض إلا بِخِمَار».
Explanation
It is not permissible for a woman of age and sound intellect to pray without a veil, meaning: with her head and neck bare. If she prays unveiled, her prayer is invalid. Here, the Prophet (may Allah's peace and blessings be upon him) says: "Allah does not accept" her prayer. This means that the lady's prayer is neither valid nor rewarded.
For these restrictions to apply, one must be of age, which, in Islamic terms, has signs: menstruation, emission of semen, growth of pubic hair, or completing fifteen years of age. Menstruation is mentioned in the Hadīth because it is a specific sign of female adulthood. It should be noted, however, that women cannot pray when they have their cycle. Texts of the Sunnah, as well as the scholarly consensus, rule against performing the prayer during menses.
Source
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