Musnad Abu Ya'la al-Mawsili 42:111

Uthman narrated to us, Jarir narrated to us, from al-A’mash, from Abu Salih, from Abu Sa’id, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "How can I be at ease while the companion of the Horn has put it to his lips, bent his forehead, and is waiting for when he will be commanded to blow?" It was said: We said, "O Messenger of Allah, what should we say on that day?" He said: "Say: 'Sufficient for us is Allah, and [He is] the best Disposer of affairs. Upon Allah we have relied.'"