Parashas Chayei Sarah · Passuk 24:63
And Yitzchak went out to meditate in the field toward evening and, looking up, he saw camels approaching.
Torah Temimah points to Berachos 26b. E-Daf · Sefaria — Berachos 26b
TT lemma: לשוח בשדה.
Yitzchak instituted the afternoon prayer, as it is said “And Yitzchak went out to converse in the field toward evening.” “Conversation” means only prayer.
Custom (Bavli — Yitzchak established Minchah)
Yitzchak established the Minchah prayer; “to converse in the field toward evening” is the Torah’s record of that institution.