Charles Ellicott Commentary 2 Samuel 22:3

"God, my rock, in him will I take refuge; My shield, and the horn of my salvation, my high tower, and my refuge; My saviour, thou savest me from violence." — 2 Samuel 22:3 (ASV)

The God of my rock. — In the psalm, My God, my rock (margin). The two expressions of the psalm are united here in one, and the recurrence of the similar expression in 2 Samuel 22:47 (though not in the psalm) indicates that this was intentional.

And my refuge, my saviour; thou savest me from violence. — These words are omitted from the psalm, being compensated for in part by the opening line there.