Musnad Ahmad 132:2

Hassan narrated to us, Ibn Lahi'ah narrated to us, he said: And Yahya ibn Ghaylan narrated to us, Rashdin narrated to us, Zabban ibn Fa'id al-Hamrawi narrated to us, from Sahl ibn Mu'adh ibn Anas al-Juhani, from his father, Mu'adh ibn Anas al-Juhani, the companion of the Prophet, may God bless him and grant him peace, from the Prophet, may God bless him and grant him peace, who said: "Whoever recites 'Say, He is God, [who is] One' until he completes it ten times, God will build for him a palace in Paradise." Umar ibn al-Khattab said: "Then we will increase [our recitation], O Messenger of God?" The Messenger of God, may God bless him and grant him peace, said: "God is more abundant and better."