Musnad Ahmad 25:754

Hassan ibn Musa narrated to us, Hammad ibn Salamah narrated to us, from ‘Ata’ ibn al-Sa’ib, from Abu Yahya, from Ibn ‘Abbas: That two men disputed before the Prophet, may Allah bless him and grant him peace. The Messenger of Allah, may Allah bless him and grant him peace, asked the claimant for evidence, but he had no evidence. So he asked the defendant to take an oath, and he swore by Allah, besides whom there is no god. The Messenger of Allah, may Allah bless him and grant him peace, said: "You have sworn, but Allah has forgiven you due to your sincerity in your saying: 'There is no god but Allah.'"