Musnad Abu Ya'la al-Mawsili 148:48

Nafi' ibn Khalid al-Tahi and Muhammad ibn Bahr narrated to us, they said: Nuh ibn Qays narrated to us, he said: 'Abd al-Rahman, the freed slave of Qays, narrated to us, from Ziyad al-Numayri, from Anas ibn Malik, from the Prophet (peace and blessings of Allah be upon him) that he said: "Whoever builds a mosque for Allah, whether it is small or large, Allah will build a house for him in Paradise."