In another version he related that a Bedouin came to the Prophet ﷺ and asked, “Messenger of Allah, what are the major sins?” He replied, “Associating partners with Allah.” The man said, “Then what?” He answered, “al-yamin al-ghamus (the perjurious oath).” I said, “What is al-yamin al-ghamus?” He replied, “It is the oath by which a person wrongfully takes the property of a fellow Muslim, meaning he swears while knowingly lying.”

[Commentary]
The author—may Allah have mercy on him—opens his chapter in Riyad al-Salihin entitled “The Stern Warning against the False Oath Used to Seize a Muslim’s Property.” A person who swears by Allah is obliged to tell the truth, whether the matter concerns himself or someone else. If he swears an oath while lying, and through that oath unlawfully takes the wealth of a Muslim, even if it is negligible, he will meet Allah on the Day of Resurrection while Allah is angry with him.

By way of illustration: someone brings a claim against another, saying, “I lent you 1,000 riyals.” The defendant replies, “You have nothing with me.” Because the claimant has no evidence, the judge tells the defendant, “Swear that he has nothing with you.” The defendant swears, “By Allah, he has nothing with me.” On that basis the judge will rule that the claimant has no right over him, for “the burden of proof is upon the claimant, and the oath is upon the one who denies.”