Abu Dawud Book 15, Hadith Number 3269.
Chapter : Not known.
Narated By Abdullah ibn Abbas : Two men brought their dispute to the Prophet (PBUH). The Prophet (PBUH) asked the plaintiff to produce evidence, but he had no evidence. So he asked the defendant to swear. He swore by Allah “There is no god but He.”
The Apostle of Allah (PBUH) said: Yes, you have done it, but you have been forgiven for the sincerity of the statement: “There is no god but Allah.”