Musannaf Ibn Abi Shayba 3:6471

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Ibn Abi Layla, from al-Hakam, from Ibrahim, he said: There are four things in which the Imam does not recite aloud: "In the name of Allah, the Most Gracious, the Most Merciful," the seeking of refuge (al-isti'adhah), "Amin," and "Our Lord, to You is the praise."