Musnad Abu Ya'la al-Mawsili 152:139

Al-A'mash narrated to us, from Abdullah ibn Murrah, from Abu al-Ahwas, who said: Abdullah ibn Mas'ud said: "That I should swear by Allah nine times that Ibn Sa'id is the Dajjal is more beloved to me than that I should swear once. And that I should swear 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. And that is because Allah took him as a Prophet and made him a martyr."