Musnad Ahmad 31:54

Sufyan narrated to us, from Mutarrif, from Atiyyah, from Abu Sa’id, from the Prophet, may Allah bless him and grant him peace, who said: "How can I be at ease when the companion of the Horn has taken the Horn into his mouth, bent his forehead, and inclined his ear, waiting to see when he will be commanded?" The Muslims said: "O Messenger of Allah, what should we say?" He said: "Say: 'Sufficient for us is Allah, and [He is] the best Disposer of affairs; upon Allah we have relied.'"