Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.5

3.2.1.5

atha jaghanena kṛṣṇājine paścāt prāṅ jānvākna upaviśati sa yatra śuklānāṃ ca kṛṣṇānāṃ ca saṃdhirbhavati tadevamanimṛśya japatyṛkṣāmayoḥ śilpe stha iti yadvai pratirūpaṃ tacilpamṛcāṃ ca sāmnāṃ ca pratirūpe stha ityevaitadāha

He then squats down behind the two skins, with his face towards the east and with bent (right) knee; and while touching them thus at a place where the white and black (hair) join, he mutters (Vâg. S. IV, 9), 'Ye are the images of the Rik and Sâman;'--an image doubtless is what is conformable: 'Ye are conformable to, the riks and sâmans' he thereby means to say.