Hadith: If anyone swears by a religion other than Islam falsely and intentionally, he is as he has said

عن ثابِت بْن الضَّحَاكِ رضي الله عنه أنه بايع رسول الله صلى الله عليه وسلم تحت الشجرة، وأن رسول الله صلى الله عليه وسلم قال: «من حلف على يمين بملة غير الإسلام، كاذبا مُتَعَمِّدًا فهو كما قال، ومن قتل نفسه بشيء عُذِّبَ به يوم القيامة، وليس على رجل نَذْرٌ فيما لا يملك» وَفِي رِوَايَة: «ولَعْنُ المؤمنِ كَقَتْلِهِ». وفي رواية: «من ادَّعَى دَعْوَى كَاذِبَةً لِيَتَكَثَّرَ بها لم يَزِدْهُ اللهُ -عزَّ وجل- إلا قِلَّةً».
[صحيح] - [متفق عليه]

Thābit ibn Ad-Dahhāk (may Allah be pleased with him) reported: I gave an oath of allegiance to the Messenger of Allah (may Allah's peace and blessings be upon him) under the tree. Then, the Messenger (may Allah's peace and blessings be upon him) said: "If anyone swears by a religion other than Islam falsely and intentionally, he is as he has said. If anyone kills himself with something, he will be punished with it on the Day of Judgment. A man cannot make a vow on something that he does not possess." Another narration reads: "Cursing a believer is like killing him." A third narration stipulates: "If anyone makes a false claim to an increase of something, Allah, the Exalted and Glorified, will only give him a decrease."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Thābit ibn Ad-Dahhāk al-Ansāri (may Allah be pleased with him) is one of those who gave an oath of allegiance to the Prophet (may Allah's peace and blessings be upon him) under the tree in Bay‘at Ar-Ridwān on the day of Hudaybiyah. He reported that the Prophet (may Allah's peace and blessings be upon him) said that if someone intentionally swears by a religion other than Islam, he would be as he swore. In other words, he would be a Jew, Christian, Zoroastrian, or a disbeliever, or he would be disassociated from Allah and His Messenger if did such and such. If he falsely makes such an oath intentionally, he would belong to the non-Islamic religion that he had ascribed himself to in the oath. And if someone kills himself with a sword, knife, bullet, or any lethal object, he will be punished with that object on the Day of Judgment. Additionally, if a person makes a vow on something that he doesn't possess, like vowing to free a slave that he does not own or to give charity from someone else's money, such a vow is null and void, because the one making the vow doesn't own what he is making the vow on. Also, cursing a believer is like killing him, because both acts violate what Allah, the Almighty, has prohibited and are considered as sins and warrant a punishment. Furthermore, if someone exaggerates and proudly claims to have what he does not possess, such as wealth, knowledge, family background, position, etc., Allah will increase him in nothing but humiliation and degradation. This is because such a person wants to elevate himself through something he doesn't have. So, he will be punished with the opposite of his objective.