Ibn ʿAbd al-Barr has transmitted, through the line of ʿAmr ibn Shuʿayb, a Prophetic report that states: “The evidence is upon the claimant, and the oath upon the one who denies—except in qasāmah.” Al-Muwaffaq commented, “This additional phrase must be acted upon, for any supplementary wording narrated by a trustworthy transmitter is accepted.”

The crux of qasāmah is that the oath is placed with whichever litigant’s position is the stronger. In ordinary lawsuits the defendant’s denial carries greater weight, because the default presumption is freedom from liability. In qasāmah, however, the claimant is supported by incriminating circumstantial evidence (lawth). Shaykh al-Islam explains: “Any indication that strengthens the claim against the defendant tips the balance in the claimant’s favor, so the oath passes to him. And because of the seriousness of qasāmah and the sanctity of blood, a single oath is not sufficient; it must be repeated fifty times.”

Imam Aḥmad said, “I apply qasāmah whenever there is an evident cause.”

Shaykh al-Islam further defined lawth as “that which makes it more likely that the claim is true.”

Al-ʿAllāmah Ibn al-Qayyim added, “This is one of the best proofs for acting upon outward indicators that create a strong presumption of the claimant’s truthfulness. On that basis he may swear the oaths, and the judge is obliged to rule accordingly, establishing the right to retaliation or blood money, even though the judge neither witnessed the act nor heard testimony.”