John Gill Commentary 2 Samuel 15:8

"For thy servant vowed a vow while I abode at Geshur in Syria, saying, If Jehovah shall indeed bring me again to Jerusalem, then I will serve Jehovah." — 2 Samuel 15:8 (ASV)

For your servant vowed a vow while I abode at Geshur in Syria ,
&c.] When at his grandfather's court there:

saying, if the Lord will bring me again indeed to Jerusalem ;
which he might be sincerely desirous of:

then I will serve the Lord ;
but it is a question whether he ever made a vow to this purpose, or concerned himself about serving the Lord; but it rather may be, this was a lie of his, now framed in order to get leave of the king to go to Hebron.