Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) issued judgments based on an oath and a witness

عن ابن عباس «أن رسول الله صلى الله عليه وسلم قَضَى بِيَمِين وشَاهِدٍ».
[صحيح] - [رواه مسلم]

Ibn ‘Abbās reported: The Messenger of Allah (may Allah's peace and blessings be upon him) issued judgments based on an oath and a witness.
Sahih/Authentic. - [Muslim]

Explanation

This Hadīth constitutes a fundamental principle in Islamic jurisprudence. It shows that the Prophet (may Allah's peace and blessings be upon him) issued judgments based on a claimant's oath and one witness if it was difficult to bring another witness. This applies to issues related to money and similar things. However, a testimony must be provided. First, the witness should testify and then the claimant should be sworn in. If only one witness is available, the situation will not be settled, but it is more likely to be in the claimant’s favor. So an oath is recommended to be taken by the party who has the strongest claim. This view is adopted by the majority of the Companions, Tābi‘is, and the three Imāms (Mālik, Ash-Shāfi‘i, and Ahmad).