The works of Ādi Śaṅkarācārya

Chāndogyopaniṣadbhāṣyam

Verse 4

वागेवर्क्प्राण सामोमित्येतदक्षरमुद्गी-थ ।तद्वा एतन्मिथुनं यद्वाक्च प्राणश्च-र्क्च साम च ॥ ५ ॥

In plain words

Speech itself is the ṛk, and prāṇa is the sāman. This syllable OM is the udgītha. Indeed, that forms a pair: speech and prāṇa, as well as ṛk and sāman.

Close to the Sanskrit

Speech indeed is the ṛk, prāṇa is the sāman, this syllable om is the udgītha. That indeed is this pair: which are speech and prāṇa, ṛk and sāman.