מדרש על סנהדרין 40:7

whence it was ruled: A king breaches (the fence of others) to make way for himself (to go to his vineyard or his field), and he may not be held back (from doing so). He may broaden roads for himself, and he is not held back from doing so. The path of the king has no limits. And all of the people take of the spoil and place it before him, and he takes the prime portion. (He chooses first and takes half of all the spoil.)