From pages 251, 252 of Ibn Sa’d’s Kitab al-Tabaqat:
….When the apostle of Allah conquered Khaibar and he had peace of mind, Zaynab Bint al-Harith, the brother of Marhab, who was the spouse of Sallam Ibn Mishkam, inquired, “Which part of the goat is liked by Muhammad?” They said, “The foreleg.” Then she slaughtered one from her goats and roasted it (the meat). Then she wanted a poison which could not fail. …. The apostle of Allah took the foreleg, a piece of which he put into his mouth. Bishr 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 foreleg; …informed me that it is poisoned. Thereupon Bishr said, “By Him who has made you great! I DISCOVERED IT FROM THE MORSEL I TOOK. Nothing prevented me from emitting it out, but the idea that I did not like to make your food unrelishing. 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.".......(bold and capital emphasis mine)
From this narration some polemical detractors came up with an argument. They claim that this is proof that the Prophet SAW was making up revelations (Nauzubillah) and had actually tasted the poison same as Bishr. Is it authentic? Can anyone help address it?