What is the oldest regulation in the world regarding a head of state?

There is a tradition of written recognitions of the rights of princes against the monarch in the middle ages. This rights were the most important limits of the power of the monarch.

For example in the Holy Roman Empire there were so called Reichsgrundgesetze (basic laws of the Empire). The first are the Concordat of Worms of 1122, the Confoederatio cum principibus ecclesiasticis of 1220, the Statutum in favorem principum of 1231/1232 and the Golden Bull of 1356.

In the Concordat of Worms the Investiture Controversy was settled. Pope Callixtus II and Emperor Henry V agreed that the Emperor had no right to elect the bishops and abbots in the Empire despite them beeing princes and vassals of the Emperor. But he had the right to formally invest them in they worldly authority, handing over the scepter.

The Statutum in favorem principum (Statute in favour of the princes) together with the Confoederatio cum principibus ecclesiasticis (Treaty with the princes of the church) of 1220 forbade the monarch to build towns and castles and guaranteed some rights of the princes, as jurisdiction and tolls.

In the Golden Bull the election of a new King (later to crown to Emperor) is organized in detail. Additionally it guaranteed the special rights of the prince-electors (Kurfürsten).

One remark: In the middle ages the idea of "setting new law" was pretty uncommon. Almost all acts were to "restore the good old law" and to write it down. Indeed most of the rights mentioned above were in practice decades before and only solemnly guaranteed by the emperor because of particular political circumstances in this documents.

I found the Code of Ur-Nammu as code of laws, where in the prologue the code defines to source of power of the king, which is derived to deities. This seems the oldest surviving code of laws issued by a king.

Although the preface directly credits the laws to king Ur-Nammu of Ur (2112–2095 BC), the actual author who had the laws written down onto cuneiform tablets is still somewhat under dispute.

...

"…After An and Enlil had turned over the Kingship of Ur to Nanna, at that time did Ur-Nammu, son born of Ninsun, for his beloved mother who bore him, in accordance with his principles of equity and truth... Then did Ur-Nammu the mighty warrior, king of Ur, king of Sumer and Akkad, by the might of Nanna, lord of the city, and in accordance with the true word of Utu, establish equity in the land; he banished malediction, violence and strife, and set the monthly Temple expenses at 90 gur of barley, 30 sheep, and 30 sila of butter. He fashioned the bronze sila-measure, standardized the one-mina weight, and standardized the stone weight of a shekel of silver in relation to one mina... The orphan was not delivered up to the rich man; the widow was not delivered up to the mighty man; the man of one shekel was not delivered up to the man of one mina."

The declaration says the king of Ur, Sumer and Akkad is de-facto word of Utu, the Sumerian Sun God, therefore it passes as a regulation of authority of the king.

Homer's Iliad describes some relationships between Greek military leaders and their peoples. For instance, it concerns

• How they divide the military loot. It seems they had some rules on what the king(s) may take and what is then distributed otherwise. For instance, king Achilles calls Briseis (a captured Troyan woman) "the most worthy gift to me from the Acheans".

• How the leaders distribute powers between themselves. It seems that Agamemnon Atreidais is the supreme leader, king who is in command of other kings in the alliance (evidently corresponding to the Greek title of Archistrategos), but his powers are challenged by Achilles, who criticizes him for not justfully distributing the loot, for not enough participation in the war and other things. He says he will not follow the unjust decisions by Agamemnon and will pull out of the war.

• How pressure is exercised on the leaders. It seems the most important decisions are taken at the councils. Agamemnon is forced to return his loot, the captured woman Chryseis to her father, a priest so to please the gods. Yet he demands and forces to hand over the substitution from another leader, Achilles.

It seems the most ancient limitations on the head of state's powers were of religious character: he should be honest and respect the gods, priests and rites.