Musnad Abu Ya'la al-Mawsili 107:19

Ahmad ibn Isa al-Tustari narrated to us, Abdullah ibn Wahb narrated to us, from Amr ibn Malik, from Ubayd Allah ibn Abi Ja'far, from Ja'far ibn Abdullah ibn al-Hakam, who said: I heard Uqbah ibn Amir saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever is thrown from his mount in the way of Allah and dies, he is a martyr."