TT #7 · Parashas Toldos · Passuk 25:23
and Hashem answered her, “Two nations are in your womb, two separate peoples shall issue from your body; one people shall be mightier than the other, and the elder shall serve the younger.”
Torah Temimah points to Megillah 6a. E-Daf · Sefaria — Megillah 6a
Rabbi Yitzchak said: If a person tells you that both Caesarea and Jerusalem are destroyed, do not believe him… as it is said, “One people shall be mightier than the other.”
Custom (Bavli — perpetual rivalry of the two nations)
The two nations cannot both flourish or both decline at the same time.