Musnad Ahmad 32:902

Waki' narrated to us, Shu'bah narrated to us, from Qatadah, from Anas, who said: "I prayed behind the Messenger of Allah (peace and blessings of Allah be upon him), and behind Abu Bakr, and Umar, and Uthman, and they would not recite aloud: 'In the name of Allah, the Most Gracious, the Most Merciful' (Bismillah al-Rahman al-Rahim)."