John Gill Commentary 2 Samuel 4:9

"And David answered Rechab and Baanah his brother, the sons of Rimmon the Beerothite, and said unto them, As Jehovah liveth, who hath redeemed my soul out of all adversity," — 2 Samuel 4:9 (ASV)

And David answered Rechab and Baanah his brother, the sons of
Rimmon the Beerothite
In a manner they did not expect:

and said to them, [as] the Lord lives, who has redeemed my soul
out of all adversity ;

spiritual and temporal, especially the latter is meant, and particularly what he had been brought into by the persecution of Saul, while living, and by those that adhered to his house since his death; which he ascribes to the Lord, and doubted not that he would still deliver him, and complete what he had designed for him, and that he needed not the assistance of such wicked hands as theirs.

The words contain the form of an oath made to testify the truth of the following narrative, concerning the man who brought the tidings of Saul's death to him, or for the certainty of what he would do to those persons for the murder of Ishbosheth.