Musnad Ahmad 31:705

'Abd al-Razzaq narrated to us, Sufyan informed us, from al-A'mash, from al-'Awfi, from Abu Sa'id al-Khudri, that the Prophet, peace and blessings of Allah be upon him, used to say: "How can I be at ease when the companion of the Horn has put the Horn to his mouth, bent his forehead, and inclined his ear, waiting for when he will be commanded?"