In Oedipus Rex, what clue does Jocasta give to Oedipus concerning the central problem of the play?

We need to remember the central dramatic irony that runs
throughout this play. Oedipus is trying to find out who has killed the former king of
Thebes, Laius, and by so doing has brought down the plague upon Thebes which is killing
its citizens. However, having pledged himself to discovering the murderer, Oedipus finds
more and more clues to suggest that it was actually him who killed Laius, his father,
and then married his mother.

In the part of the play you
are referring to, Jocasta tells her husband/son that an oracle told Laius that he would
be killed by his own son. Jocasta then says that Laius had his son abandoned to die on a
lonely mountainside, so therefore, Laius could not have been killed by his son. Instead
Laius was killed "By marauding strangers where three highways meet". It is this precise
description of the manner and location of Laius's death that forces Oedipus to remember
something that greatly disturbs him. Note how he responds to
Jocasta:

How
strange a shadowy memory crossed my mind,

Just now while
you were speaking; it chilled my
heart.

Oedipus doesn't fully
remember yet, but as the play progresses and he assembles more and more clues, he
discovers that the prophecy was fulfilled - it was he who killed his father and is the
cause of the punishment of Thebes for this unnatural act of patricide and then the act
of marrying his mother.