Tribute, Spoils, and Rulership (Kitab Al-Kharaj, Wal-Fai' Wal-Imarah)
Dawud :: Book 19 : Hadith 2926
Narrated Aisha, Ummul Mu'minin:
The Apostle of Allah (peace_be_upon_him) said: When Allah has a good purpose for a ruler, He appoints for him a sincere minister who reminds him if he forgets and helps him if he remembers; but when Allah has a different purpose from that for him. He appoints for him an evil minister who does not remind him if he forgets and does not help him if he remembers.