Musannaf Ibn Abi Shayba 34:298

Yazid narrated to us, Abu al-Fadl Khalid ibn Riyāh informed us, Abu al-Siwār al-Adawī narrated to us, from Jundub ibn ‘Abd Allāh, who said: "The first part of the son of Adam to become foul-smelling when he dies is his abdomen, so do not put anything into it except what is good."