From Sahih Muslim (Hadith 1189):
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 (ﷺ). Thereupon he said: You have not been fair to me for I have heard from the Messenger of Allah (ﷺ) 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
Reference: Muslim 1189