Musnad Abu Ya'la al-Mawsili 153:164

Abu Khaythama narrated to us, Waki' narrated to us, Hanzala al-Jumahi narrated to us, from Salim, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If your women seek permission from you to go to the mosque, then give them permission."