2 Samuel 15:7-10

7And after four years, Absalom said to the king, “Please let me go and pay my vow which I have made to the Lord at Hebron [my birthplace].

8For your servant made a vow while I lived at Geshur in Aram (Syria), ‘If the Lord will in fact bring me back to Jerusalem, then I will serve the Lord [by offering a sacrifice of thanksgiving].’ ”

9And [David] the king said to him, “Go in peace.” So he arose and went to Hebron.

10But Absalom sent spies throughout all the tribes of Israel, saying, “As soon as you hear the sound of the trumpet, you shall say, ‘Absalom is king in Hebron.’ ”