TT points to R. Yochanan: when that righteous man reached Dan his strength ebbed, because he saw his descendants who would later serve idolatry at Dan.
English of Core Text: R. Yochanan said: once that righteous man reached Dan, his strength ebbed — he saw his descendants who would in the future serve idolatry at Dan.
On the daf / in context: The place-name Dan is read proleptically; the future sin of the northern cult weakens Avraham already in the pursuit.
Type of Learning
Custom
What Is Learned
Even a future descendant’s idolatry can diminish the strength of the ancestor who passes the place.