Sunni and Shia sources mentioned that the Prophet Muhammad (SAWA) was poisoned and did not die a natural death. Among Sunni sources mentioned this fact: al-Mustadrak 'Alal Saheehain by al-Haakim al-Neesabouri.
'Musnad Ahmad ibn Hanbal from Ibn Mas'ood.
The question is who put the poison to poison the Prophet? Some claim that it was a Jewish woman who sent roasted meat to the Prophet in Khaibar but this claim can not stand because all who have eaten from that poisoned meat died and Allah informed the Prophet that it is poisoned and he did not eat from it. How can he dies after three years of that?
Those who poisoned the Prophet wanted to use that old incident to cover their act.