Musnad Abu Ya'la al-Mawsili 160:77

Abu Bakr ibn Abi Shaybah narrated to us, Hatim ibn Isma'il narrated to us, from Humayd ibn Sakhr, from al-Maqburi, from Abu Hurayrah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever comes to this mosque of mine, and he has not come except for a good thing that he learns or teaches, then he is in the position of the mujahid in the way of Allah. And whoever comes for other than that, then he is in the position of the man who looks at the property of someone else."