'Took an Oath' in the Bible

Then all the people came and encouraged David to eat food while it was still day. But David took an oath saying, "God will punish me severely if I taste bread or anything whatsoever before the sun sets!"

But Abishai the son of Zeruiah came to David's aid, striking the Philistine down and killing him. Then David's men took an oath saying, "You will not go out to battle with us again! You must not extinguish the lamp of Israel!"