For God, in making promises to Abraham, swore by himself, (because he had no one greater by whom he might swear)
Other versions
King James Version
For when God made promise to Abraham, because he could swear by no greater, he sware by himself,English Standard Version
For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself,New International Version
When God made his promise to Abraham, since there was no one greater for him to swear by, he swore by himself,