This narration is mentioned in the Musnad of Ahmad bin Hanbal in which Abdullah ibn Mas'ood said that I don't mind or in fact I am ready to take oath and repeat it nine times that the prophet was killed rather than taking oath one time that he was not killed.
Musnad Ahmad bin Hanbal volume 1 page 408
Our prominent Ulamah have mentioned many authentic hadith that the Prophet was poisoned. Among them is:
· Sheikh Al Mufid in his book Al-Muqni'ah page 456
· Sheikh Al Toosi in his book Al Tahdhib vol 2 page 6
· Al Fattal Al Nisaburi in his book Al Rawdat page 71
· Al Allamah Al Hilli in his book Al Tahrir vol 2 page 118
· Al Shahid Al Awal in his book Al-Durus vol 2 page 6
· Allamah Al Majlisi in his book Mir'at Al Uqol vol 5 page 174
Ibne-Abil Hadeed Al Moathazalee a well known non-Shia scholar mentioned in his book Sharh Nahj Al-Balagha that the Prophet was poisoned and that he passed away as a Shahid (martyr) vol 10 page 221.
There is a narration from Imam Jaffer as Sadiq (as) that Imam Hassan (as) told his family members "I will die by poison as the Prophet Mohammad (saw) was poisoned."
Al-Khara'ij wal-Jara'ih vol 1 page 241
Among the Sunni scholars Al Hakim Al Nishapuri is very well known who compiled the famous book Al-Mustadrak alaa al-Sahihain he narrated from Al Sha'bi "by Allah the Prophet was poisoned."
It is not the death but it is the martyrdom of the Prophet Mohammad (peace be upon him and his Holy Progeny).