Amir al-Mu'minin
When the Byzantine empress Irene was deposed in 802, Nikephoros I became emperor and refused to pay tribute to Harun, saying that Irene should have been receiving the tribute the whole time. News of this angered Harun, who wrote a message on the back of the Roman emperor's letter and said "In the name of God the most merciful, From Amir al-Mu'minin Harun ar-Rashid, commander of the faithful, to Nikephoros, dog of the Romans. Thou shalt not hear, thou shalt behold my reply". After campaigns in Asia Minor, Nikephoros was forced to conclude a treaty, with humiliating terms.