Sahih Muslim Book 42, Hadith Number 7110.

Chapter : Excellence of Building Mosques.

Mahmud b. Labid reported that ‘Uthman b. ‘Affan decided to rebuild the mosque (of Allah’s Apostle in Medina) but the people did not like this idea and they wished that it should be preserved in the same (old) form. Thereupon he (Hadrat ‘Uthman) said: I heard Allah’s Messenger (may peace be upon him) as saying: He who builds a mosque for Allah, Allah would build for him (a house) in Paradise like it.