Midrasch zu Schemuel II 8:1

... “Moab is my washbasin…” (Tehillim 60:10) When Israel entered into their land in order to inherit it, the Holy One forbid them to conquer these three nations, as it says “Do not distress the Moabites…” (Devarim 2:9) So too regarding Edom it is written “You shall not provoke them…” (Devarim 2:5) From where do we learn that they were not to conquer the land of the Pelishtim? Because it is written “God did not lead them [by] way of the land of the Philistines for it was near…” (Shemot 12:17) The oath which Avraham swore to Avimelech was still near in time, “And now, swear to me here by God, that you will not lie to me or to my son or to my grandson…” (Bereshit 21:23) His grandson was still alive. In the future the Holy One will permit Israel to conquer all three, as it says “And they shall fly of one accord against the Philistines in the west, together they shall plunder the children of the East; upon Edom and Moab shall they stretch forth their hand, and the children of Ammon shall obey them.” (Yeshayahu 11:14) And it is translated as ‘they will join shoulder to shoulder as one to wipe out the Phillistines.’ Therefore it says “…Philistia, join me…” (Tehillim 60:10), Edom and Moav are their occupation as it says “Moab is my washbasin; on Edom I will throw my lock…” (ibid.)

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).