Judges 1:36

And the coast of the Amorites was from the going up to Akrabbim, from the rock, and upward.

Cross-reference

Numbers 34:4 describes the same 'ascent of Akrabbim' as the southern border — confirming the Amorite boundary.

Joshua 15:3 explicitly names the 'ascent of Akrabbim' — identical phrase to the Amorite border here.