Musnad Abu Ya'la al-Mawsili 117:91

Abu al-Rabi' narrated to us, Hatim ibn Maymun narrated to us, Thabit narrated to us, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever recites 'Say, He is Allah, [who is] One' two hundred times in a day, one thousand and five hundred good deeds will be written for him, unless he has a debt."