Musnad Ahmad 26:230

Aswad ibn Amir narrated to us, Zuhayr narrated to us, from Abu Ishaq, from Sa’d or Sa’id ibn Iyad, from Abdullah ibn Mas’ud, who said: "The most beloved part of the sheep to the Messenger of Allah, peace and blessings of Allah be upon him, was the foreleg. And he used to believe that there was poison in the foreleg of the sheep." And we used to believe that it was the Jews who poisoned him.