Śatapatha Brāhmaṇa ("of a hundred paths") 5.3.2.2

5.3.2.2

svarbhānurha vā āsuraḥ | sūryaṃ tamasā vivyādha sa tamasā viddho na vyarocata tasya somārudrāvevaitattamo 'pāhatāṃ sa eṣo 'pahatapāpmā tapati tatho evaiṣa etattamaḥ praviśatyetaṃ vā tamaḥ praviśati yadayajñiyānyajñena prasajatyayajñiyānvā etadyajñena prasajati śūdrāṃstvadyāṃstvattasya somārudrāvevaitattamo 'pahataḥ so 'pahatapāpmaiva dīkṣate tadyacvetāyai śvetavatsāyai payasi śṛto bhavati kṛṣṇaṃ vai tamastattamo 'pahanti tasyaiṣaiva śvetā śvetavatsā dakṣiṇā

Now, once upon a time, Svarbhânu, the Âsura, struck the sun with darkness, and stricken with darkness he did not shine. Soma and Rudra removed that darkness of his; and freed from evil he burns yonder. And in like manner does that (king) thereby enter darkness,--or darkness enters him,--when he puts those unworthy of sacrifice in contact with the sacrifice; and he does indeed now put those unworthy of sacrifice--either Sûdras or whomever else--in contact with the sacrifice. It is Soma and Rudra who remove that darkness of his; and freed from evil he becomes consecrated. And as to why it is cooked in milk from a white (cow) which has a white calf,--darkness is black: that darkness he removes. The sacrificial fee for this (oblation) is a white (cow) which has a white calf.