Yahya related to me from Malik that he had heard that Abdullahibn Umar said, "The Messenger of Allah, may Allah bless him and granthim peace, said, 'Do not forbid the female slaves of Allah from (goinginto) the mosques of Allah.' "
USC-MSA web (English) reference: Book 14, Hadith 12
Arabic reference: Book 14, Hadith 469