2 Samuel 15:7-10

7At the end of 40 years Absalom said to the king, “Please let me go to Hebron and pay my vow which I have vowed to Adonai.

8For your servant vowed a vow while I was still living at Geshur in Aram saying, ‘If Adonai will indeed bring me back to Jerusalem, then I will serve Adonai.’”

9So the king said to him, “Go in shalom.” Then he rose and went to Hebron.

10But Absalom sent spies to all the tribes of Israel to say, “As soon as you hear the sound of the shofar, then you are to say, ‘Absalom has become king in Hebron!’”