Hebrews 6:13

For when God made promise to Abraham, because he could swear by no greater, he sware by himself,
— Hebrews 6:13, King James Version

In other translations

World English Bible (WEB)

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

American Standard Version (ASV)

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

Young's Literal Translation (YLT)

For to Abraham God, having made promise, seeing He was able to swear by no greater, did swear by Himself,