2 Samuel 10:9-11

9When Joab saw that he would have to fight on both the front and the rear, he chose some of Israel’s elite troops and placed them under his personal command to fight the Arameans in the fields.

10He left the rest of the army under the command of his brother Abishai, who was to attack the Ammonites.

11“If the Arameans are too strong for me, then come over and help me,” Joab told his brother. “And if the Ammonites are too strong for you, I will come and help you.