Musnad Ahmad 30:2613

Waki' narrated to us, he said: Ibn Abi Dhi'b narrated to us, from Sa'id ibn Abi Sa'id, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A woman shall not travel the distance of a full day, except with a mahram."