Musannaf Ibn Abi Shayba 3:1784

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Shu’bah, he said: I asked al-Hakam, Hammad, and Abu Ishaq, and they said: "Recite 'In the name of Allah, the Most Gracious, the Most Merciful' in every rak'ah."