Musannaf Ibn Abi Shayba 3:6475

Hushaym narrated to us, he said: Husayn and Mughirah narrated to us, from Ibrahim, who said: The Imam conceals "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."