Мидраш к Бамидбар 22:27

... [T]he Holy One has pity on the honor of the creatures/b’riyot (here meaning “people”) and knows their needs. And He shut the mouth of the beast/b’heimah, for if she would speak, they could not make her serve or stand up to her /bah, for [there was] this silent one from the animals (the ass) and this wise one from the sages (Bil`am) – [and] when she spoke he could not stand up to her.