The works of Ādi Śaṅkarācārya

Chāndogyopaniṣadbhāṣyam

Bhashya

Bhashya

नक्षत्राणामधिपतिश्चन्द्रमा अत स साम ॥

In plain words

The moon is the lord of the stars, and therefore he is the sāman.

Close to the Sanskrit

The moon is the lord of the stars, hence he is the sāman.