'Took an Oath' in the Bible

All the people came to urge David to eat food while it was still day; but David took an oath, saying, “May God do so to me, and more also, if I taste bread or anything else before the sun sets.”

But Abishai, the son of Zeruiah, came to his help, and, turning on the Philistine, gave him his death-blow. Then David's men took an oath, and said, Never again are you to go out with us to the fight, so that you may not put out the light of Israel.