Musannaf Ibn Abi Shayba 3:1759

Hushaym narrated to us, he said: Husayn and Mughirah narrated to us, from Ibrahim, who said: The Imam shall recite "In the name of Allah, the Most Gracious, the Most Merciful," the seeking of refuge (isti'adhah), "Amin," and "Our Lord, to You is the praise" silently.