Al-Kāfi

Hadith 995
major-sins
5ـ يُونُسُ عَنْ حَمَّادٍ عَنْ نُعْمَانَ الرَّازِيِّ قَالَ سَمِعْتُ أَبَا عَبْدِ الله (عَلَيهِ السَّلام) يَقُولُ مَنْ زَنَى خَرَجَ مِنَ الإيمَانِ وَمَنْ شَرِبَ الْخَمْرَ خَرَجَ مِنَ الإيمَانِ وَمَنْ أَفْطَرَ يَوْماً مِنْ شَهْرِ رَمَضَانَ مُتَعَمِّداً خَرَجَ مِنَ الإيمَانِ.
5. Yunus has narrated from Hammad from Nu’man al-Razi who has said the following: “I heard Abu ‘Abdillah (a.s.) saying, ‘Whoever commits fornication suffers the loss of his belief, one who drinks wine suffers the loss of belief and one who breaks intentionally the fast in the month of Ramadhan suffers the loss of his belief.’”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī