Musnad Ahmad 28:31

Isma'il narrated to us, Sulayman al-Taymi narrated to us, from Aslam al-'Ijli, from Bishr ibn Shughaf, from Abdullah ibn 'Amr, who said: A Bedouin said: "O Messenger of Allah, what is the Trumpet (al-Sur)?" He said: "A horn (qarn) into which it is blown."