Anti-Islamists claims the following:
"It is on the heels of this assassination that Ibn Sunayna, a Jewish merchant, was assassinated. With the success of the five conspirators, Muhammad said, "Kill any Jew that falls into your power." Shortly afterwards, Muhayyisa b. Masud leapt upon and killed Ibn Sunayna, with whom Muhayyisa had some social and business relations. However, Muhayyisa’s elder brother, not a Muslim at the time, beat the assassin, the younger brother, saying, "You enemy of God, did you kill him when much of the fat on your belly comes from his wealth?" Muhayyisa retorted that if Muhammad had ordered even the elder brother’s assassination, he would have carried it out. The elder was impressed: "By God, a religion which can bring you to this is marvelous!" And he became a Muslim. That is, the elder brother implies that Muhammad must be a great leader and worthy of devotion if he commands such lethal reverence and deadly obedience from his followers.
Then Muhayyisa wrote a poem that celebrates such obedience. "I would smite his [the elder brother’s] neck with a sharp sword, / A blade as white as salt from polishing. / My downward stroke never misses its mark." Advancing religious violence, these lines in the poem show how deadly poetry could be, and they match the Muslim’s poem against Abu Afak (no. 4, above): "a hanif gave you a thrust in the night." Kab’s poem, it should be recalled, was far milder. These poems that a Westerner reads in the early Islamic source are jarring. It seems the early Muslim authors of the documents relish inserting them into their books."
Source: Ibn Ishaq p. 369 / 534
Firstly its quite funny because they sort of kill off their own argument with the following:
"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 marvellous!' and he became a Muslim."
If one was to believe that the above mentioned killing did happen because of the order of Prophet Muhammad and that command was completely unjust and absolutely unwarranted for (as we are led to believe), then could the person who had questioned the killer would have accepted Muhammed as the "Messenger of God" and become a Muslim? This shows that the charactar of the prophet was not in question.
Despite this however and the fact that this story is mentioned in Sunan Abu Dawood, it is weak and unreliable. Concerning isnad (i.e. chain of reporters), this Hadith was narrated by servant of Zaid Ibn Thabet on authority of daughter of Muhayyisah. Servant of Zaid is Muhammad Ibn Ibi Muhammad and he is unreliable, and daughter of Muhayyisah is unknown. Concerning matn (i.e. text), it says that Prophet Muhammad (peace be upon him) ordered to kill all Jews which is illogical even if Christian missionaries want to believe it!
Because the Jews had a treaty with Muslims and there was no evidence that Muslims indulged in killing any Jew other than this Hadith. Moreover, Ibn Hesham himelf who edited the work of Ibn Ishaq suggests that the incident of Huwayyisah and Muhayyisah occurred during slaughter of Bani-Qurayzah, not after murder of Ka’b(1). Needless to say, there is no such thing as “Kill any Jew that come under your power”.
We reject the above because of the following:
1) The chain of narration for the above hadith is unreliable and has a missing person in it
2) Any hadith which contradicts the teachings of the Quran is rejected by the Muslims. The Quran clearly teaches us killing innocent people is a sin. It clearly teaches us Jews and Christians are people of the book. It clearly teaches us not to break a covenant or a promise. The above hadith contradicts all these Quranic teachings.
For those who refuse to accept the Muslim position on this hadith, and those who continue to suggest this actually happened - there however is also evidence that this hadith was reported from the incident at banu qurayzah, which woud mean the killing may have been for the deception and treason of the Jews at that time.
Either way you look at it, the allegations are false.
(1) As-Sirat-un-Nabawiyyah, Volume 3, page 18