Volume 3, Book 47, Number 786:
Narrated Anas bin Malik (radiallaahu `anhu):
A Jewess brought a poisoned (cooked) sheep for the Prophet (sallallaahu `alayhi wasallam) who ate from it. She was brought to the Prophet (sallallaahu `alayhi wasallam) and he was asked, “Shall we kill her?” He said, “No.” I continued to see the effect of the poison on the palate of the mouth of Allah’s Apostle (sallallaahu `alayhi wasallam).
Al-Nawawi said: It is as if the poison still left some trace of blackness etc.
The name of this woman was Zaynab bint al-Haarith, the wife of Salaam ibn Mashkam, one of the leaders of the Jews.
The reports differ as to whether or not she was killed. It seems that the Prophet (peace and blessings of Allaah be upon him) did not kill her at first, but when Bishr ibn al-Bara’ ibn Ma’roor died as a result of the effects of this food, then he executed her as a qisaas punishment.
Al-Bukhaari (5777) narrated that Abu Hurayrah said: When Khaybar was conquered, a roasted poisoned sheep was presented to the Prophet as a gift (by the Jews). The Messenger of Allaah (peace and blessings of Allaah be upon him) said, “Let all the Jews who have been here, be assembled before me.” The Jews were gathered and the Messenger of Allaah (peace and blessings of Allaah be upon him) said, “Will you now tell me the truth, if I ask you about something?” They replied, “Yes.” He asked, “Have you poisoned this sheep?” They said, “Yes.” He asked, “What made you do that?” They said, “We wanted to know if you were a liar in which case we would have got rid of you, and if you are a Prophet then the poison would not harm you.”
The Prophet (peace and blessings of Allaah be upon him) used to feel ill because of this food, and he would be treated with cupping for that.
That had an impact in causing his death, so he (peace and blessings of Allaah be upon him) died as a martyr (shaheed), as Ibn Mas’ood (may Allaah be pleased with him) said:
“If I were to swear by Allaah nine times that the Messenger of Allaah (peace and blessings of Allaah be upon him) was killed, that is more beloved to me than swearing once, because Allaah made him a Prophet and made him a martyr.” Narrated by Ahmad, 3617. The editors said, its isnaad is saheeh according to the conditions of Muslim.
Al-Sindi said: The words “he was killed” mean by the poison in the meat of the sheep’s foreleg that he ate, when the effects of that appeared when he was dying.
Quoted from Haashiyat al-Musnad, 6/116.
Al-Bukhaari narrated in his Saheeh, in a mu’allaq report, and al-Haakim narrated in his Mustadrak in a mawsool report, that ‘Aa’ishah (may Allaah be pleased with her) said: “The Prophet (peace and blessings of Allaah be upon him) used to say, in the illness which would be his last, ‘O ‘Aa’ishah, I still feel the pain of the food that I ate in Khaybar, and this time I feel that my aorta is being cut from that poison.”
The aorta is the vein that is towards the back and is connected to the heart; if it is cut then the person will die.
The conquest of Khaybar took place in Muharram or Rabee’ al-Awwal of the year 7 AH. So this event took place four years before the Prophet (peace and blessings of Allaah be upon him) died.
Taken from IslamQA.