Sahih (Sahih Muslim)
وَحَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا هِشَامٌ، قَالَ قَالَ أَبِي فَأُخْبِرْتُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لَقَدْ حَكَمْتَ فِيهِمْ بِحُكْمِ اللَّهِ عَزَّ وَجَلَّ " .
It has been narrated on the authority of Hisham (who learnt it from his father) that the Messenger of Allah (ﷺ) said (to Sa'd):You have adjudged their case with the judgment of God. the Exalted and Glorified