Chandogya Upanishad 1.7.4

1.7.4

atha yad etad akṣnaḥ śuklaṃ bhāḥ saivark \
atha yan nīlaṃ paraḥkṛṣṇaṃ tat sāma \
tad etad etasyām ṛcy adhyūḍhaṃ sāma \
tasmād ṛcy adhyūḍhaṃ sāma gīyate \
atha yad evaitad akṣṇaḥ śuklaṃ bhāḥ saiva sā \
atha yan nīlaṃ paraḥkṛṣṇaṃ tad amaḥ \
tat sāma \\107.4\

4. Now, the Ṛic is the bright shining of the eye; the Sāman is the dark, the ultra-black. This Sāman rests upon that Ṛic. Therefore the Sāman is sung as resting upon the Ṛic. *sā* is the bright shining of the eye; *ama* is the dark, the ultra-black. That makes *sāma*.