Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.3.9

4.2.3.9

taṃ yadapurorukkaṃ gṛhṇāti | ukthaṃ hi purorugṛgghi purorugṛgghyukthaṃ sa yadevainamukthebhyo vigṛhṇāti teno hāsyaiṣa puroruṅmānbhavati tasmādapurorukkaṃ gṛhṇāti

And the reason why he draws this (graha) without a puroruk, is that the puroruk is praise, (being) a Rik, and the song of praise is Rik; and in that he divides it for recitations, thereby indeed it becomes possessed of a puroruk: hence he draws it without a puroruk.