It was narrated that Abu Hurairah said:
"I heard the Messenger of Allah say: 'Whoever comes to this mosque of mine, and only comes for a good purpose, such as to learn or to teach, his status is like that of one who fights in Jihad in the cause of Allah. Whoever comes for any other purpose, his status is that of a man who is keeping an eye on other people's property.'"
English reference: Vol. 1, Book 1, Hadith 227
Arabic reference: Book 1, Hadith 232