Musnad Abu Ya'la al-Mawsili 149:28

Mus'ab ibn 'Abd Allah narrated to us, Hisham ibn 'Abd Allah ibn 'Ikrimah narrated to me, from Hisham ibn 'Urwah, from his father, from 'Aishah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Seek provision in the hidden depths of the earth."