2 Samuel 15:7
And it came to pass after forty years, that Absalom said unto the king, I pray thee, let me go and pay my vow, which I have vowed unto the LORD, in Hebron. And it came to pass after forty years, that Absalom said unto the king, I pray thee, let me go and pay my vow, which I have vowed unto the LORD, in Hebron.
Absalom asked David for permission to go to Hebron to pay a vow to the Lord that he claimed to have made while in exile in Geshur. The request was no sincere act of piety but a cloak of religious devotion assumed to conceal his traitorous designs for revolt, made only after the minds of the people had been prepared and his agents had been sent out. It was a crowning act of hypocrisy that blinded David, who took it as evidence of piety and dismissed Absalom with a blessing. It was calculated to establish the confidence of the people in order to lead them into rebellion against the king whom God had chosen.