Parashas Ki Savo · Passukim 26:17–18
And Hashem has distinguished you today to be for Him a treasured people…
Torah Temimah points to related material on the reciprocal form (and parallel discussions of mutual commitment).
English: “You have distinguished / He has distinguished you” — a covenant of reciprocity: you will not exchange Me for another god, and I will not exchange you for another nation.
Custom (diyuk from the mutual verbal form).
The verse records a two-sided commitment of exclusivity between Hashem and Israel.