The works of Ādi Śaṅkarācārya

Chāndogyopaniṣadbhāṣyam

Bhashya

Bhashya

पशून् न निन्देत्, तद्व्रतम् ॥

In plain words

One must never speak ill of cattle; that is the observance.

Close to the Sanskrit

He should not criticize cattle, that is the vow.