What confirms that Israel's future land will extend to the Red Sea and the river of Egypt?

Les explains that in Genesis 15, God appeared to Abraham and literally went through the rites of transferring the title deed to Abraham. After doing that, in verse 18, the Lord made a covenant with Abraham, saying that unto his seed He has given this land from the river of Egypt to the great river Euphrates. Les notes that this river of Egypt is not the Nile, but another river a little east of the Nile, down in that neck of the woods. He says this land extends clear out through present-day Iraq down to the Persian Gulf. That entire area will be Israel's homeland when the Kingdom is set up, because she has never had it yet. David and Solomon got part of it, but not near all of it. So the day is coming when the land of Israel will actually cover that whole Middle East. Les also points out that we know it will cover all that area to the south and across to the tip of the Red Sea, and over to this river of Egypt, by virtue of the tribes that are listed. So the Genesis 15 covenant and the listing of the tribes confirm the future boundaries.