The following is from the draft english translation of the Arabic book مقدمة الدستورأو الأسباب الموجبة له (Introduction to the constitution and the evidences that make it obligatory) published by Hizb ut-Tahrir 1382 Hijri (1963 CE). Please refer to the original Arabic for accurate meanings. Please note some of the adopted opinions of the Hizb have changed since the time the book was published so any of the adopted literature published after this book which contradicts what is mentioned in this book abrogates those specific points.
Article 16: The ruling system is a system of unity and not a federalist system
The sound ruling system is a system of unity and nothing else; because Shari'ah has decreed such a system and forbidden any other system. It has been reported on the authority of Abdullah Ibn Amr Ibn Al-A'as that he heard the Messenger of Allah (saw) say: "Whoever pledged allegiance to an Imam, giving him the clasp of his hand and the fruit of his heart shall obey him as long as he can, and if another comes to dispute with him, you must strike the neck of that man." It has also been reported on the authority of Abu Said Al-Khudri that the Messenger of Allah (saw) said: "If a pledge of allegiance (Baya'a) has been taken for two Khalifahs kill the latter of them." The evidential aspect of these two Ahadith is that the first Hadith reflects the obligation of obeying the one with whom the Khilafah has been contracted, and if another came to dispute this Khilafah with him, he then should be fought and killed if he did not retract. Hence, the Hadith explains that he who disputes the authority of the Khalifah in the Khilafah he should be killed; this is tantamount to the prohibition of dividing the State. The Hadith also exhorts the Muslims to prevent, even by force, any attempt at dividing the State or staging a breakaway from it.. As for the second Hadith, it indicates that in case the State had no head, i.e. no Khalifah and the Khilafah were given to two persons, then the latter should be killed; the more reason if it were given to more than two. This is tantamount to the prohibition of dividing the State. It stipulates the prohibition of turning the State into states. It should rather be one single state. Hence, the ruling system in Islam is a system of unity and not a federalist system. Any other than the system of unity is categorically and conclusively forbidden. That is why this article has been drafted.