It is narrated from Sayyiduna Ibn Amr (may Allah be pleased with him) that a Bedouin came to the court of the Messenger and asked, "O Messenger of Allah! What is the Sur (Trumpet)?" He replied: "It is a horn in which it will be blown."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6805