Musnad Ahmad 32:1971

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 'In the name of Allah, the Most Gracious, the Most Merciful' aloud."