Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.067A.
Section : The excellence of building mosques.
‘Uthman reports that the Prophet sallallahu alehi wasallam said: “Whoever builds for Allah a mosque, seeking by it Allah’s grace, Allah will build for him a house in paradise.” This is related by al-Bukhari and Muslim.
Ibn ‘Abbas reports that the Prophet sallallahu alehi wasallam said: “Whoever builds for Allah a mosque, even if it be tiny, like a bird’s nest, Allah will build for him a house in paradise.” This is related by Ahmad, Ibn Hibban, and al-Bazzar with a sahih chain.