Good Manners and Form (Al-Adab)
Ṣaḥīḥ al-Bukhārī 6105
مَنْ حَلَفَ بِمِلَّةٍ غَيْرِ الإِسْلاَمِ كَاذِبًا فَهْوَ كَمَا قَالَ، وَمَنْ قَتَلَ نَفْسَهُ بِشَىْءٍ عُذِّبَ بِهِ فِي نَارِ جَهَنَّمَ، وَلَعْنُ الْمُؤْمِنِ كَقَتْلِهِ، وَمَنْ رَمَى مُؤْمِنًا بِكُفْرٍ فَهْوَ كَقَتْلِهِ
man ḥalafa bimillatin ghayri al-islāmi kādhiban fahwa kamā qāla, waman qatala nafsah bishayʾin ʿudhiba bihi fī nāri jahannama, walaʿnu al-muʾmini kaqatlihi, waman ramā muʾminan bikufrin fahwa kaqatlihi
Transliteration generated from the Arabic vowel marks
The full narration
Narrated Thabit bin Ad-Dahhak:The Prophet (ﷺ) said, "Whoever swears by a religion other than Islam (i.e. if he swears by saying that he is a non-Muslim in case he is telling a lie), then he is as he says if his oath is false and whoever commits suicide with something, will be punished with the same thing in the (Hell) fire, and cursing a believer is like murdering him, and whoever accuses a believer of disbelief, then it is as if he had killed him
Source
Ṣaḥīḥ al-Bukhārī · 6105