One cold winter day, the Gaon, Rabbi Nosson Adler, was traveling together with his esteemed talmid, the Chasam Sofer. The horses pulling the wagon were trudging through the heavy snow with great difficulty. Suddenly, one of the horses collapsed and died, and the second horse was not strong enough to pull the wagon alone. The wagon driver, having no other choice, starting walking to the nearest village to obtain an additional horse. The two venerated passengers waited in the wagon.
Eventually, the wagon driver returned, leading a.....donkey. When Rav Nosson saw the donkey, he descended from the wagon, and began to dance happily in the snow. "Rebbi," asked the Chasam Sofer, "Why are you so happy?"
"Don't you see?" asked Rav Nosson. "The wagon driver brought a donkey instead of a horse. Who would have ever thought that I would merit fulfilling the mitzva of, "Do not plow with an ox and donkey together"? At home in Frankfurt, I never imagined that I would merit fulfilling this commandment. Now that Baruch Hashem, I merited it, I am filled with joy!"
The passengers instructed the wagon driver to return the donkey, and he brought back a horse in its place. (Ukarasa Lashabbos Oneg)