Al-Kāfi - Volume 3 > Causes of Death and that Believing People May Die by any Manner of Dying
Hadith #6

6ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ أَوْ غَيْرِهِ عَنْ عَلِيِّ بْنِ حَدِيدٍ عَنِ الرِّضَا (عَلَيْهِ الْسَّلام) قَالَ أَكْثَرُ مَنْ يَمُوتُ مِنْ مَوَالِينَا بِالْبَطَنِ الذَّرِيعِ.

6. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad or someone other than him from Ali ibn Hadid from who has said the following: “Al-Rida, ‘Alayhi al-Salam, has said that most of our followers die from widespread stomach (internal) illness.”