Musnad Ahmad 1153:1816

'Affan narrated to us, Hammad narrated to us, he said: Qatadah informed us, from Muhammad ibn Sirin, from Safiyyah bint al-Harith, from 'Aishah, that the Prophet, peace and blessings of Allah be upon him, said: "The prayer of a menstruating woman [i.e., a woman who has reached the age of puberty] is not accepted unless she wears a head covering (khimar)."