Musnad Ahmad 1244:4

Umayyah ibn Khalid narrated to us, he said: Muhammad ibn Abdullah ibn Muslim, the nephew of al-Zuhri, narrated to us, from his uncle al-Zuhri, from Humayd ibn Abd al-Rahman, from his mother, that she said: The Messenger of Allah, may Allah bless him and grant him peace, said: " 'Say, He is Allah, [who is] One' is equivalent to one-third of the Qur'an."