Musnad Ahmad 27:930

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, peace and blessings of Allah be upon him. The Messenger of Allah, peace and blessings of Allah be upon him, 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, peace and blessings of Allah be upon him, said: "You have indeed done it, but you have been forgiven by your sincerity in saying 'There is no god but Allah'."