Musnad Ahmad 26:231

Aswad narrated to us, Israel narrated to us, from Abu Ishaq, from Sa’d ibn ‘Iyad, from Ibn Mas’ud, who said: "Indeed, some eloquence is magic." He said: And we used to believe that the Messenger of Allah (peace and blessings of Allah be upon him) was poisoned in the foreleg of a sheep; the Jews poisoned it.