Praise be to Allah.
Her fast is not valid and she has to make up that day, because the basic principle is that the menses was still ongoing, and her beginning to fast when she was not sure that she had become pure was embarking upon an act of worship when she was not sure that the conditions of its validity were met, and this means that it is not valid. End quote.
Shaykh Muhammad ibn ‘Uthaymeen.
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