Musnad Ahmad 26:323

'Abd al-Razzaq narrated to us, Sufyan informed us, from al-A'mash, from 'Abd Allah ibn Murrah, from Abu al-Ahwas, from 'Abd Allah, who said: "That I should swear nine times that the Messenger of Allah (peace and blessings of Allah be upon him) was killed [by murder] is more beloved to me than that I should swear once that he was not killed. And that is because Allah made him a prophet and took him as a martyr." Al-A'mash said: I mentioned that to Ibrahim, and he said: "They used to believe that the Jews poisoned him and Abu Bakr."