Musnad Ahmad 1156:71

Yahya ibn Ghaylan narrated to us, he said: Rishdin narrated to us, 'Amr narrated to me, from Abu al-Samh, from al-Sa'ib, the freed slave of Umm Salamah, from Umm Salamah, from the Messenger of Allah, may Allah bless him and grant him peace, that he said: "The best of the mosques for women are the innermost parts of their houses."