Imam Al-Nawawi’s Riyad-us-Saliheen
Prohibition of Addressing somebody as ‘The King of Kings’
1724. Abu Hurairah (May Allah be pleased with him) said: The Prophet (sallallaahu alayhi wa sallam) said, “The most disgraceful man near Allah is a man who calls himself (or likes others to call him) Malikul-Amlak (i.e., king of kings).”
[Al-Bukhari and Muslim].