Musnad Ahmad 25:1149

Asbat narrated to us, Mutarrif narrated to us, from Atiyyah, from Ibn Abbas, regarding His saying: {And when the trumpet is blown} [Al-Muddaththir: 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, and has bent his forehead, listening for when he will be commanded, so that he may blow?" The companions of Muhammad 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.'"