Musnad Ahmad 3:36

'Abd al-Kabir ibn 'Abd al-Majid Abu Bakr al-Hanafi narrated to us, 'Abd al-Hamid—meaning Ibn Ja'far—narrated to us, from his father, from Mahmud ibn Labid, from 'Uthman ibn 'Affan, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "Whoever builds a mosque for Allah, the Mighty and Majestic, Allah will build for him the like of it in Paradise."