According to Emmerich de Vattel, in his The Law of Nations or The Principles of Natural Law, first published in 1758, he writes:
It is a much discussed question whether a sovereign must observe the ordinary laws of war in dealing with rebellious subjects who have openly taken up arms against him. A flatterer at court or a cruel tyrant will immediately answer that the laws of war is not made for rebels who deserve only death.
To which he replies:
In order to understand clearly what conduct a sovereign should observe we must first of all recall that the rights of the sovereign are derived wholly [and not in part] from the rights of the State itself or of civil society, from the duties entrusted to him, and from the obligation he is to watch over the welfare of the nation, to procure its greatest happiness and to maintain order, peace and justice in the country.
Q. Is this stipulated in any convention of International law in more or less this form?
I don't expect the exact wording to be followed, but a very similar sentiment and thought.