Musnad Ahmad 1060:50

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur, from Hilal ibn Yasaf, from Rabi' ibn Khuthaym, from 'Amr ibn Maymun, from a woman, from Abu Ayyub, from the Prophet, may Allah bless him and grant him peace, that he said: "Say, 'He is Allah, [who is] One' is equivalent to a third of the Qur'an."