Sahih Muslim Book 4, Hadith Number 1085.

Chapter : Forbiddance to build mosques on the graves and decorating them with pictures and forbiddance to use the graves as mosques.

Mahmud b. Labid reported: When ‘Uthman b. ‘Affan intended to build the mosque (of the Prophet) the people did not approve of it. They liked that it should be kept in the same state. Thereupon he said: I heard the Messenger of Allah (may peace be upon him) say: He who built a mosque for Allah, Allah would build a house for him like it in Paradise.