Midrash for I Samuel 7:13

When David reigned, he desired to enter the land of the Philistines, but he was unable (to do so) because of the power of the sign of the covenant oath of || Isaac, until he had taken from them the sign of the covenant of Isaac's oath, as it is said, "And David took the bridle of the cubit out of the hand of the Philistines" (2 Sam. 8:1), as it is written, "So the Philistines were subdued, and they came no more within the border of Israel" (1 Sam. 7:13).