Musnad Ahmad 1153:1149

Abu Kamil and 'Affan narrated to us, they said: Hammad narrated to us, from Qatadah—'Affan 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)."