Musnad Ahmad 30:3345

Ammar ibn Muhammad—who is the nephew of Sufyan—narrated to us, from Ibrahim, from Abu Iyad, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the likeness of knowledge that does not benefit is like the likeness of a treasure from which nothing is spent in the way of Allah."