Kommentar zu Bereschit 31:49

והמצפה אשר אמר AND MIZPAH (literally, the Mizpah); BECAUSE HE SAID etc . — The word והמצפה and the Mizpah must refer to some well-known place bearing this name (for there are several places named Mizpah) it refers to the Mizpah which is in Mount Gilead — as it is written (Judges 11:29) “and he passed over Mizpah of Gilead”. Why was it also called Mizpah? Because each of them said to the other, “The Lord watch (יצף) between me and thee that thou shouldst not violate this covenant”.

AND MITZPAH. Rashi comments that the meaning of the verse is: “And the Mitzpah which is on Mount Gilead, as it is written, And he passed over Mitzpah of Gilead.²⁵³Judges 11:29. Why was it called Mitzpah? Because each of them said to the other, The Eternal ‘yitzeph’ (watch).” And if so, Mitzpah is the name of a high place on top of the mountain. In my opinion, however, Mitzpah is the stone which Jacob set up for a pillar²⁵⁴Verse 45 here. and is connected with the above verse: Therefore was the name of it called Galed,²⁵⁵Verse 48 here. and he also called it the Mitzpah because he said, The Eternal watch between me and thee. It is possible that this stone is Mitzpah of Gilead,²⁵³Judges 11:29. for the place was always called by the name of this stone.

והמצפה, this lookout is referred to in Judges 11,29.