It was narrated from 'Aishah that: The Prophet entered upon her, and a freed slave girl of hers concealed herself. The Prophet asked: Have her periods begun? She said: Yes. He tore a piece of his turban and said: Cover your head with this.
It was narrated from 'Aishah that: The Prophet said: Allah does not accept the prayer of a woman who menstruates (i.e., an adult woman) except with a head cover.