“The Sunna and path of the Prophet constitute a proof of the necessity for establishing government. First, he himself established a government, as history testifies. He engaged in the implementation of laws, the establishment of the ordinances of Islam, and the administration of society. He sent out governors to different regions; both sat in judgment himself and appointed judges; dispatched emissaries to foreign states, tribal chieftains, and kings; concluded treaties and pacts; and took command in battle. In short, he fulfilled all the functions of government. Second, he designated a ruler to succeed him, in accordance with divine command. If God Almighty, through the Prophet, designated a man who was to rule over Muslim society after him, this is in itself an indication that government remains a necessity after the departure of the Prophet from this world. Again, since the Most Noble Messenger promulgated the divine command through his act of appointing a successor, he also implicitly stated the necessity for establishing a government.”

Islam and Revolution, Writings and Declarations of Imam Khomeini, Translated and Annotated by Hamid Algar, Mizan Press, Berkley, pp. 41.
Islamic government