Musnad Ahmad 647:1

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Jami' ibn Shaddad, from Abdullah ibn Yasar, who said: I was sitting with Sulayman ibn Surad and Khalid ibn 'Arfatah, and they were intending to follow the funeral of a person who died of a stomach ailment. One of them said to his companion: "Did the Messenger of Allah, peace and blessings of Allah be upon him, not say: 'Whoever is killed by his stomach will not be punished in his grave'?" He replied: "Yes."