2 Samuel 10:9-11

9When Joab saw the battle lines before him and behind him, he selected some of the best men of Israel and arrayed them against the Arameans. 10And he placed the rest of the troops under the command of his brother Abishai, who arrayed them against the Ammonites.

11“If the Arameans are too strong for me,” said Joab, “then you will come to my rescue. And if the Ammonites are too strong for you, then I will come to your rescue.