Musnad Ahmad 30:2665

Yazid narrated to us, he said: Muhammad—meaning Ibn Ishaq—informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Banu Abd al-Muttalib, purchase yourselves from Allah. O Safiyyah, aunt of the Messenger of Allah, and O Fatimah, daughter of the Messenger of Allah, purchase yourselves from Allah, for I cannot avail you against Allah in anything. Ask me of my wealth whatever you wish."