Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.3.9

3.1.3.9

tamabhyanakti | śīrṣato 'gra ā pādābhyāmanulomam mahīnām payo 'sīti mahya iti ha vā etāsāmekaṃ nāma yadgavāṃ tāsāṃ vā etatpayo bhavati tasmādāha mahīnām payo 'sīti varcodā asi varco me dehīti nātra tirohitamivāsti

He anoints him from the head down to the feet in accordance with the tendency of the hair, with the text (Vâg. S. IV, 3), 'Thou art the sap of the great ones.' The 'great ones,' doubtless, is one of the names of those cows, and their sap indeed it is: therefore he says, 'thou art the sap of the great ones.' 'Thou art life-giving: give me light!' There is nothing obscure in this.