Musnad Ahmad 26:589

'Abd al-Rahman narrated to us, from Sufyan, from al-A'mash, from 'Abdullah ibn Murrah, from Abu al-Ahwas, from 'Abdullah, who said: "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 that he was not killed. And that is because Allah, the Almighty and Majestic, made him a prophet and took him as a martyr." He said: So I mentioned that to Ibrahim, and he said: "They used to believe and say: 'The Jews poisoned him, and Abu Bakr, may Allah be pleased with him.'"