Musnad Ahmad 30:1482

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu Sakhr narrated to us, from al-Maqburi, from Abu Hurairah, from the Messenger of Allah, may Allah bless him and grant him peace, who said: "Whoever enters this mosque of ours to learn good, or to teach it, is like the mujahid in the way of Allah. And whoever enters it for other than that, is like the one looking at what does not belong to him."