According to the author of Chronicles, when King Tou of Hamath heard that King David had beaten King Hadarezer's army, he learned about David's strength and success. This information might have made King Tou feel interested or even worried about how strong and powerful David had become.
1 Chronicles 18:9: Now when Tou king of Hamath heard how David had smitten all the host of Hadarezer king of Zobah; [Tou: also called, Toi]
This verse refers to the same event but differs in details about the account and sequence of events related to the interactions with King Toi of Hamath.
2 Samuel 8:9: When Toi king of Hamath heard that David had smitten all the host of Hadadezer,