Sunan Abu Dawud, Book 19, Number 2996:

Narrated Muhayyisah:
The Apostle of Allah (peace_be_upon_him) said: If you gain a victory over the men of Jews, kill them. So Muhayyisah jumped over Shubaybah, a man of the Jewish merchants. He had close relations with them. He then killed him. At that time Huwayyisah (brother of Muhayyisah) had not embraced Islam. He was older than Muhayyisah. When he killed him, Huwayyisah beat him and said: O enemy of Allah, I swear by Allah, you have a good deal of fat in your belly from his property.

Further details about this incident are given by Ibn Ishaq in his Sirat Rasul Allah as translated by A. Guillaume, The Life of Muhammad, page 369:

THE AFFAIR OF MUHAYYISA AND HUWAYYISA
The apostle said, 'Kill any Jew that falls 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 marvellous!' and he became a Muslim.

I was told this story by a client of B. Haritha from the daughter of Muhayyisa from Muhayyisa himself.

Muhayyisa composed the following lines on the subject:

My mother's son blames me because if I were ordered to kill him
I would smite his nape with a sharp sword,
A blade white as salt from polishing.
My downward stroke never misses its mark.
It would not please me to kill you voluntarily
Though we owned all Arabia from north to south.

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