Musnad Ahmad 25:319

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Jabir, from 'Ammar, from Sa'id ibn Jubayr, from Ibn 'Abbas, from the Prophet, may Allah bless him and grant him peace, that he said: "Whoever builds a mosque for Allah, even if it is like the nest of a sandgrouse for its eggs, Allah will build a house for him in Paradise."