That the Messenger of Allah (ﷺ) said: "Whoever swears by a religion other than Islam while lying, then he is as he said."
The Book on Vows and Oaths - Jami' at-Tirmidhi
Hadith Reference: Jami` at-Tirmidhi 1543
Hadith Text
"Whoever swears by a religion other than Islam while lying, then he is as he said."
Scholarly Commentary
This hadith contains a grave warning against swearing by false deities or religions. The phrase "while lying" indicates the person knows the falsity of what they swear by, yet deliberately chooses it over Allah.
Scholars explain that such an oath constitutes shirk (associating partners with Allah) because it attributes divine authority to something other than Allah. The consequence "he is as he said" means he will be accounted as having left Islam if he dies upon this belief.
Imam al-Tirmidhi classified this hadith as hasan sahih (good and authentic). The ruling applies when one intentionally swears by other religions recognizing their falsehood, not out of ignorance. The person must immediately repent and reaffirm tawhid.
Legal Ruling
This oath is invalid and sinful. One must seek forgiveness and renew their shahadah. No expiation (kaffarah) suffices for shirk - only sincere repentance. The scholars unanimously prohibit swearing by anything except Allah.