Musar על בבא מציעא 63:7

Included in this matter is to not cause pain to any creature, even animals, and to show mercy and concern towards them. Likewise scripture states: "the righteous man knows the soul of his beast" (Mishlei 12:10) (Rashi-what his beast needs), and according to some of our sages (Shabbat 128b), to cause pain to an animal is a biblical prohibition, while to others it is at least a Rabbinical prohibition.