Śatapatha Brāhmaṇa ("of a hundred paths") 5.4.3.19

5.4.3.19

atha vārāhyā upānahā upamuñcate | agnau ha vai devā ghṛtakumbham praveśayāṃ cakrustato varāhaḥ sambabhūva tasmādvarāho meduro ghṛtāddhi sambhūtastasmādvarāhe gāvaḥ saṃjānate svamevaitadrasamabhisaṃjānate tatpaśūnāmevaitadrase pratitiṣṭhati tasmādvārāhyā upānahā upamuñcate

He then puts on shoes of boar's skin. Now the gods once put a pot of ghee on the fire. Therefrom a boar was produced: hence the boar is fat, for it was produced from ghee. Hence also cows readily take to a boar: it is indeed their own essence (life-sap, blood) they are readily taking to. Thus he firmly establishes himself in the essence of the cattle: therefore he puts on shoes of boar's skin.