Musannaf Ibn Abi Shayba 3:5687

Abu Bakr narrated to us, he said: Waki' narrated to us, from Muhammad ibn Qays, from Ali ibn Mudrik, from Ibrahim, from al-Rabi' ibn Khuthaym, who said: "Is one of you unable to recite a third of the Qur'an while he is bowing or prostrating: 'Say, He is Allah, [who is] One, Allah, the Eternal Refuge'?"