Musannaf Ibn Abi Shayba 21:467

Asbat ibn Muhammad narrated to us, from Mutarrif, from Atiyyah, from Ibn Abbas, regarding the saying of Allah the Exalted: "And when the trumpet is blown" (74:8). He said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "How can I be at ease when the companion of the Horn has put the Horn to his mouth, bent his forehead, and is listening for when he will be commanded so that he may blow?" The companions of the Prophet (peace and blessings of Allah be upon him) said: "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.'"