Kauṣītaki (Śāṅkhāyana) Brāhmaṇa 17.5.8

17.5.8

tad āhur ataḥ kasmād uttamāt pratīhārād ūrdhvam āhūya sāmnā śastram upasaṃtanvanti iti / (soma: sodaśin & atiratra)

xvii. 6. They recite strophes and antistrophes in Gāyatrī; the Gāyatrī is light; night is the evil, the darkness; thus thereby they smite away the evil, the darkness. They recite with repetitions, for so do the Sāman chanters sing; (they say) 'According as it is sung, is it recited.' Then they say 'Why after the final Pratihāra do they call and link the Çastra with the Sāman?'