When God made His promise to Abraham, by whom did He swear, according to Hebrews 6?

Answer: Himself

Since God had no one greater to swear by, He swore by Himself, saying, "I will surely bless you and multiply your descendants."

“When God made His promise to Abraham, since He had no one greater to swear by, He swore by Himself, saying, “I will surely bless you and multiply your descendants.””— Hebrews 6:13-14 (BSB)