Musnad Ahmad 1153:1815

Bahz narrated to us, Hammad ibn Salamah narrated to us, he said: Qatadah and Yunus narrated to us, he said: Hammad narrated to us, from Qatadah, from Muhammad ibn Sirin, from Safiyyah bint al-Harith, from Aisha, that the Messenger of Allah, 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)."