Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / Hadith Number: 4369
Through the same chain of transmitters Shu'ba has narrated the same tradition in which he says that the Messenger of Allah (may peace be upon him) said (to Sa'd): You have adjudged according to the command of God. And once he said: you have adjudged by the decision of a king.