Musnad Ahmad 32:1948

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, and Hajjaj said: Shu'bah narrated to me, he said: I heard Qatadah narrating from Anas ibn Malik, he said: "I prayed with the Messenger of Allah, peace and blessings of Allah be upon him, and with Abu Bakr, and Umar, and Uthman, and I did not hear any one of them saying: 'In the name of Allah, the Entirely Merciful, the Especially Merciful' (Bismillah al-Rahman al-Rahim)."