Sahih Muslim 1768
Sahih Muslim · Book 32: The Book of Jihad and Expeditions · Permissibility of fighting those who break a treaty; Permissibility of letting besieged people surrender, subject to the judgement of a just person who is qualified to pass judgement
Through the same chain of transmitters Shu'ba has narrated the same tradition in which he says that the Messenger of Allah (ﷺ) said (to Sa'd):
وَحَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ مَهْدِيٍّ ، عَنْ شُعْبَةَ ، بِهَذَا الإِسْنَادِ وَقَالَ فِي حَدِيثِهِ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لَقَدْ حَكَمْتَ فِيهِمْ بِحُكْمِ اللَّهِ " . وَقَالَ مَرَّةً " لَقَدْ حَكَمْتَ بِحُكْمِ الْمَلِكِ " .
You have adjudged according to the command of God. And once he said: you have adjudged by the decision of a king.