Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.4.15

2.2.4.15

tāmu hāgnirabhidadhyau | mithunyanayā syāmiti tāṃ sambabhūva tasyāṃ retaḥ prāsiñcattatpayo 'bhavattasmādetadāmāyāṃ gavi satyāṃ śṛtamagnerhi retastasmādyadi kṛṣnāyāṃ yadi rohiṇyāṃ śuklameva bhavatyagnisaṃkāśamagnerhi retastasmātprathamadugdhamuṣṇam bhavatyagnerhi retaḥ

Now, Agni coveted her: 'May I pair with her,' he thought. He united with her, and his seed became that milk of hers: hence, while the cow is raw, that milk in her is cooked (warm); for it is Agni's seed; and therefore also, whether it be in a black or in a red (cow), it is ever white, and shining like fire, it being Agni's seed. Hence it is warm when first milked; for it is Agni's seed.