Hebrews 6:15

For when God made a promise to Abraham, since he could swear by none greater, he swore by himself,

saying, "Surely blessing I will bless you, and multiplying I will multiply you.

For men indeed swear by a greater one, and in every dispute of theirs the oath is final for confirmation.

In this way God, being determined to show more abundantly to the heirs of the promise the immutability of his counsel, interposed with an oath;