Musnad Ahmad 30:2296

Qutaybah narrated to us, he said: Hatim ibn Isma'il narrated to us, from Humayd al-Kharrat, from al-Maqburi, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever comes to this mosque of mine, and does not come except for a good that he learns or teaches, he is in the position of the mujahid in the way of Allah. And whoever comes to it for other than that, he is in the position of a man looking at the property of someone else."