Musannaf Ibn Abi Shayba 33:262

Ali ibn Mushir narrated to us, from al-Shaybani, from Bashir ibn Amr, who said: When Umar ibn al-Khattab came to al-Sham, a pack horse (baradhun) was brought to him, so he mounted it. When it shook him, he dismounted from it and struck its face, and said: "May Allah make you ugly, and may He make ugly the one who taught you this."