Musnad Ahmad 834:6

Yahya ibn Sa'id narrated to us, from Uthman ibn Ghiyath, Abu Na'amah narrated to me, from the son of Abdullah ibn Mughaffal, he said: Our father, when he heard one of us saying, "In the name of Allah, the Most Gracious, the Most Merciful," would say: "Hush, hush. I prayed behind the Messenger of Allah (may Allah bless him and grant him peace), and Abu Bakr, and Umar, and I did not hear any one of them saying: 'In the name of Allah, the Most Gracious, the Most Merciful.'"