Hadithcore
Sahih Muslim, 1769 b
sahih

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.

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 ‏

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References2 variants
In-Book Reference
Book 32, Hadith 80
USC-MSA web (English) reference
Book 19, Hadith 4371 (deprecated numbering scheme)