He traced it to the Prophet ﷺ, who said, "The most despicable name before Allah on the Day of Resurrection will be that of a man who called himself King of Kings."
حدثنا أحمد بن حنبل حدثنا سفيان بن عيينة عن أبي الزناد عن الأعرج
يبلغ به النبي ﷺ قال أخنع اسم عند الله تبارك وتعالى يوم القيامة رجل تسمى ملك الأملاك
Muslim narrated it in the Book of Manners, in the chapter on the prohibition of naming oneself "King of Kings" or "Sovereign of Sovereigns," hadith no. 2143, from Ahmad, and it is found in the Musnad, 2/244. Bukhari also narrated it in the Book of Manners, in the chapter on the names most hateful to Allah, hadith no. 6206, through the hadith of Sufyan bin Uyayna, with the same wording.