Narrated Abdullah ibn Abbas: Two men brought their dispute to the Prophet (ﷺ). The Prophet (ﷺ) 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 Messenger of Allah (ﷺ) said: Yes, you have done it, but you have been forgiven for the sincerity of the statement: "There is no god but Allah." Abu Dawud said: This tradition means that he did not command him to make atonement
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا حَمَّادٌ، أَخْبَرَنَا عَطَاءُ بْنُ السَّائِبِ، عَنْ أَبِي يَحْيَى، عَنِ ابْنِ عَبَّاسٍ، : أَنَّ رَجُلَيْنِ، اخْتَصَمَا إِلَى النَّبِيِّ صلى الله عليه وسلم فَسَأَلَ النَّبِيُّ صلى الله عليه وسلم الطَّالِبَ الْبَيِّنَةَ، فَلَمْ تَكُنْ لَهُ بَيِّنَةٌ فَاسْتَحْلَفَ الْمَطْلُوبَ فَحَلَفَ بِاللَّهِ الَّذِي لاَ إِلَهَ إِلاَّ هُوَ، فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم : " بَلَى قَدْ فَعَلْتَ، وَلَكِنْ قَدْ غُفِرَ لَكَ بِإِخْلاَصِ قَوْلِ لاَ إِلَهَ إِلاَّ اللَّهُ " . قَالَ أَبُو دَاوُدَ : يُرَادُ مِنْ هَذَا الْحَدِيثِ أَنَّهُ لَمْ يَأْمُرْهُ بِالْكَفَّارَةِ .
sunnah.com