Mishkat al-Masabih 15 Oaths and Vows كتاب الإيمان والنذور 5

وَعَنْ ثَابِتِ بْنِ الضَّحَّاكِ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ حَلَفَ عَلَى مِلَّةٍ غَيْرِ الْإِسْلَامِ كَاذِبًا فَهُوَ كَمَا قَالَ وَلَيْسَ عَلَى ابْنِ آدَمَ فِيمَا لَا يَمْلِكُ وَمَنْ قَتَلَ نَفْسَهُ بِشَيْءٍ فِي الدُّنْيَا عُذِّبَ بِهِ يَوْمَ الْقِيَامَةِ وَمَنْ لَعَنَ مُؤْمِنًا فَهُوَ كَقَتْلِهِ وَمَنْ قَذَفَ مُؤْمِنًا بِكُفْرٍ فَهُوَ كَقَتْلِهِ وَمَنِ ادَّعَى دَعْوَى كَاذِبَةً لِيَتَكَثَّرَ بِهَا لَمْ يَزِدْهُ اللَّهُ إِلَّا قِلَّةً»

Thabit b. ad-Dahhak reported God’s Messenger as saying, “If anyone swears by a religion other than Islam falsely he is like what he has said.* A son of Adam may not take a vow about something which he does not possess, and if anyone kills himself with something in this world he will be punished with it on the day of resurrection. If anyone curses a believer it is like murdering him, if anyone charges a believer with infidelity it is like murdering him, and if anyone makes a false claim to gain much thereby God will give him less instead of more.” (Bukhari and Muslim.) *Opinions differ about the meaning. Some say it means he is a liar, others that he is an infidel.