TT #772 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:13
You shall not have in your bag two kinds of weights, a large and a small.
Torah Temimah points to Sifrei.
TT lemma: אֶבֶן וָאָבֶן גְּדוֹלָה וּקְטַנָּה.
גְּדוֹלָה וּקְטַנָּה – גְּדוֹלָה שֶׁהִיא מַכְחֶשֶׁת אֶת הַקְּטַנָּה, שֶׁלֹּא יְהֵא נוֹטֵל בַּגְּדוֹלָה וּמַחֲזִיר בַּקְּטַנָּה.
English: “Large and small” — a large one that falsifies the small one, so that he should not take with the large and give back with the small.
On the daf / in context: The prohibition is directed against keeping two sets of weights for fraudulent use.
Derash defining the prohibited pair of weights.
One may not keep a large weight and a small weight for the purpose of cheating in measure.