It was narrated from Ibn 'Abbas (رضي الله عنه) that

Two men referred a dispute to the Prophet (ﷺ). The Messenger of Allah (ﷺ) asked the claimant for proof, but he did not have any proof, so he asked the defendant to swear an oath, and he swore by Allah, besides Whom there is no other god. The Messenger of Allah (ﷺ) said: “You have sworn an oath, but Allah has forgiven you because of your sincerity in saying, There is no god but Allah.”