Sahih Muslim Book 4, Hadith Number 1084.

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

Ubaidullah al-Khaulini reported: ‘Uthman b. ‘Affan listened to the opinion of the people (which was not favourable) when he rebuilt the mosque of the Messenger of Allah (may peace be upon him). Thereupon he said: You have not been fair to me for I have heard from the Messenger of Allah (may peace be upon him) saying: He who built a mosque for Allah, the Exalted, Allah would build for him a house in Paradise. Bukair said: I think he (the Holy Prophet) said: While he seeks the pleasure of Allah (by building the mosque). And in the narration of Ibn ‘Isa (the words are): “(a house) like that (mosque) in Paradise.”