Musnad Ahmad 647:2

Bahz narrated to us, Shu'bah narrated to us, Jami' ibn Shaddad informed me, he said: I heard Abdullah ibn Yasar, he said: Sulayman ibn Surad and Khalid ibn 'Arfatah were sitting. He said: Then it was mentioned that a man died of a stomach ailment. One of them said to his companion: "Did you not hear, or has it not reached you, that the Messenger of Allah, peace and blessings of Allah be upon him, said: 'Whoever is killed by his stomach, he will never be punished in his grave'?" The other said: "Yes."