The Jews of Madina tried to assasinate Muhammed on a number of occasions. On one occasion, they tried to poison his meal. The reason was because they did not believe in the Prophet Muhammed, his cause, and the fact that he was a messenger of God. They believed (and it was mentiond in the Torah) that there will come a last Prophet after Jesus, but they rejected Muhammed because he was from another tribe and was not from the Jewish tribes of Madina.