Kilayim, Chapter 8, Mishnah 3. We continue with the laws of forbidden mixtures of animals, and with the prohibition of driving or riding two animals of two different species together.
"HaManhig sofeg es ha'arba'im":
A person who leads two animals of two different species together - whether for work or simply while walking, when they are tied to one another - transgresses the prohibition and "absorbs the forty," meaning he receives lashes. Although in practice the number is thirty-nine lashes, the Sages used the term "forty" as the usual way of referring to lashes.
"VehaYoshev bakaron":
Regarding the one who sits in a wagon drawn by two different species of animals, the Tannaim disagree:
According to the first Tanna: Even the one sitting in the wagon absorbs the forty, since he too contributes to the work that the animals are doing.
According to Rabbi Meir: He exempts him - the one sitting in the wagon does nothing, and he does not contribute to the act of kilayim, and therefore he is exempt from lashes.
"VehaShelishis shehukshar laretzu'os assur":
This phrase has two explanations:
A third animal that was tied to the straps of the object being pulled is forbidden. For example: two horses pulling a wagon, and alongside them another species, a donkey, is tied into the arrangement. Even though the horses are the ones doing most of the pulling, since part of it is also accomplished by the strength of the donkey, it is forbidden.
A third wagon: even the person sitting in the third wagon, according to the first Tanna, transgresses the prohibition of kilayim and receives lashes, even though this wagon is almost merely dragged along with all the others.
In summary: In this Mishnah we learned that one who drives two animals of two different species absorbs the forty; regarding the one who sits in the wagon, the first Tanna holds him liable because he contributes to the work, while Rabbi Meir exempts him because he does nothing; and even a third animal that was tied to the straps - or the one sitting in the third wagon - is forbidden, since they too have a share in the pulling.