Of Mules and Mussar

Daf Yomi learners have become very familiar with the Rashbam. His commentary replaced Rashi’s for most of Bava Basra.
At the end of Eruvin, the Mordechai (528) recounts an interesting incident involving the Rashbam. The Rashbam, known for his pious demeanor and unwavering humility, consistently kept his gaze lowered as a mark of his restraint and focus on spiritual matters. On one particular day, he was about to board a horsedrawn wagon, which also included a mule in the team, a scenario that presented a significant issue in Jewish law. The riding of such a mixed team would violate the prohibition against klayim, a term that refers to the prohibition against mixing different species in acts of labor.
The Mordechai ponders whether this particular violation fell under the category of a Torah commandment or was merely a Rabbinic caution.