Rashi on Deuteronomy 22:10/רש"י על דברים כ״ב:י׳

1לא תחרש בשור ובחמור THOU SHALT NOT PLOUGH WITH AN OX AND WITH AN ASS [TOGETHER] — The same law applies to any two different kinds of animals in existence; the same law applies also to merely driving them together (when not ploughing) whilst they are yoked together as a pair carrying any load (Sifrei Devarim 232:1-2; cf. Kilayim 8:2 and 3; Bava Metzia 8b).