From Ibn Ishaq's biography of Mohammad, the earliest account of his life.
The apostle said, “Kill any Jew that fails into your power.” Thereupon Muhayyisa b. Mas’ud leapt upon Ibn Sunayna, a Jewish merchant with whom they had social and business relations, and killed him. Huwayyisa was not a Muslim at the time though he was the elder brother. When Muhayyisa killed him Huwayyisa began to beat him, saying, “You enemy of God, did you kill him when much of the fat on your belly comes from his wealth?” Muhayyisa answered, “Had the one who ordered me to kill him ordered me to kill you I would have cut your head off.” He said that this was the beginning of Huwayyisa’s acceptance of Islam. The other replied, “By God, if Muhammad had ordered you to kill me would you have killed me?” He said, “Yes, by God, had he ordered me to cut off your head I would have done so.” He exclaimed, “By God, a religion which can bring you to this is marvelous!” and he became a Muslim.