Genesis 42:15

Hereby ye shall be proved
Whether spies, or not, namely, by producing their youngest brother, said to be at home with his father: by the life of Pharaoh ye shall not go forth hence, except your
youngest brother come hither:
the phrase, "by the life of Pharaoh", seems to be the form of an oath, as it was common with many nations, especially with the Scythians, who used to swear by the royal throne {a}, and the Romans, in later times, by the life, health, and genius of their emperor; and this custom of swearing by the life of their king, or by his head, continued with the Egyptians, as Aben Ezra says, unto his times; though some take this to be a wish or prayer for the life of Pharaoh, and render it, "may Pharaoh live" ^F2, or, at most, but a strong asseveration, that as dear as the life of Pharaoh was to him, so surely they should not stir from the place where they were, unless their youngest brother Benjamin was brought thither.