Charles Spurgeon Commentary Genesis 22:15-16

"And the angel of Jehovah called unto Abraham a second time out of heaven, and said, By myself have I sworn, saith Jehovah, because thou hast done this thing, and hast not withheld thy son, thine only son," — Genesis 22:15-16 (ASV)

And the angel of the LORD called unto Abraham out of heaven the second time, and said, By myself have I sworn, saith the LORD,

Because he could swear by no greater, he sware by himself.