From Sahih Muslim (Hadith 5610):
Abu Huraira reported Allah's Messenger (ﷺ) as saying:The vilest name in Allah's sight is Malik al-Amidh (King of Kings). The narration transmitted on the authority of Shaiba (contains these words): There is no king but Allah, the Exalted and Glorious. Sufyan said: Similarly, the word Shahinshah (is also the vilest appellation). Ahmad b. Hanbal said: I asked Abu 'Amr about the meaning of Akhna. He said: The vilest
Reference: Muslim 5610