A seen in the following Hadith:
The apostle of Allah took the shoulder, a piece of which he put into his mouth. Bishr [ibn al-Bara] took another bone and put it into his mouth. When the apostle of Allah ate one morsel of it, Bishr ate his, and other people also ate from it. Then the apostle of Allah said, “Hold back your hands! because this shoulder informed me that it is poisoned.” Thereupon Bishr said, “By Him who has made you great! I realised it from the bite I took. The only reason I didn’t spit it out was that I didn’t like to spoil your appetite. When you had eaten what was in your mouth, I did not like to save my life after yours, and I also thought you would not have eaten it if there was something wrong.”
Muhammad’s companions dragged Zeynab back into his presence demanding an explanation. She replied: “I was determined to kill you because of what you did to my people. You killed my father, my uncle and my husband. I thought that if you were a [real] prophet, the poison would not harm you; but if you were only a[n ordinary] king, I would relieve everyone of you.”
So what does this mean? The woman was right? The prophet didn't know if the lamb was poison? I know this is bad source but you might wanna take a look at the article that gave me my doubts: http://www.answering-islam.org/Silas/mo-death.htm