Charles Ellicott Commentary 1 Samuel 1:26

"And she said, Oh, my lord, as thy soul liveth, my lord, I am the woman that stood by thee here, praying unto Jehovah." — 1 Samuel 1:26 (ASV)

O my lord, as thy soul liveth. —“This oath is peculiar to the Books of Samuel, in which it occurs six times, and to the Books of Kings, in which, however, it is found only once. The similar oath, as Pharaoh liveth (by the life of Pharaoh), occurs in Genesis 42:15; and as the Lord liveth is found almost exclusively in the books of which Judges is the first and 2 Kings the last, being especially frequent in the Books of Samuel. This accords with the fact that the age of the Judges and Saul was characteristically the age of vows.” — Speaker’s Commentary.