Musnad Ahmad 26:70

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abdullah ibn Murrah, from Abu al-Ahwas, from Abdullah, who said: "That I should swear by Allah nine times that the Messenger of Allah (peace and blessings of Allah be upon him) was killed as a martyr is more beloved to me than that I should swear once [that he was not]. And that is because Allah, the Mighty and Majestic, took him as a Prophet and made him a martyr."