Rashi on Deuteronomy 25:13/רש"י על דברים כ״ה:י״ג

1אבן ואבן [THOU SHALT NOT HAVE IN THY BAG] DIVERSE STONES — i.e., weights.

2גדולה וקטנה A GREAT AND A SMALL — i.e. thou shalt not have a large one that contradicts the small one, — that one must not buy (lit., take) goods by the larger and sell (lit., give it back) by the smaller (Sifrei Devarim 294:1).