Musnad Ahmad 30:3683

Abdullah ibn Yazid narrated to us, Haywah narrated to us, Abu Sakhr informed me that Sa’id ibn Abi Sa’id al-Maqburi informed him that he heard Abu Hurayrah saying: He heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever enters this mosque of ours, learning good or teaching it, is like the mujahid in the way of Allah. And whoever enters it for other than that is like one looking at what does not belong to him."