Musnad Ahmad 1153:2207

Yunus narrated to us, Hammad narrated to us, from Qatadah, from Ibn Sirin, from Safiyyah bint al-Harith, from Aisha, from the Prophet, peace and blessings of Allah be upon him, that he 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)."