https://www.btstories.com/death-of-david-joab/

Before David died, he gave Solomon instructions on how he should rule as king. He said, “To be successful, you must be strong and brave, and always walk in the ways of God’s commandments.

“Kill Joab when the time is right. He murdered two commanders of Israel—Abner, Saul’s commander, and Amasa, his cousin. He didn’t kill them on the battlefield. Both of these men trusted him because it was a time of peace. Don’t let him grow old peacefully.