"You shall not plow with an ox and donkey together"Deuteronomy 22:10.

It is forbidden to cause animals of two species to work together, e.g., to have them jointly plow, thresh or pull a wagon.

Biblically, this prohibition only applies if one of the animals is kosher (e.g., an ox) and the other of a non-kosher species (e.g., a donkey). The Sages, however, extended this prohibition to apply to any two species.