13 For when God gave the promise to Abraham, since He had no one greater to swear by, He swore by Himself,
Hebrews 6:13 Cross References - WNT
Luke 1:73
73 The oath which He swore to Abraham our forefather,
Hebrews 6:16-18
16 For men swear by what is greater than themselves; and with them an oath in confirmation of a statement always puts an end to a dispute.
17 In the same way, since it was God's desire to display more convincingly to the heirs of the promise how unchangeable His purpose was,
18 He added an oath, in order that, through two unchangeable things, in which it is impossible for Him to prove false, we may possess mighty encouragement--we who, for safety, have hastened to lay hold of the hope set before us.