Rashi on Genesis 31:49/רש"י על בראשית ל״א:מ״ט

1והמצפה 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”.

2כי נסתר WHEN WE ARE ABSENT, and shall not see (be able to watch) one another.