Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.3.7

3.1.3.7

athāgreṇa śālāṃ tiṣṭhannabhyaṅkte | arurvai puruṣo 'vācito 'narurevaitadbhavati yadabhyaṅkte gavi vai puruṣasya tvaggorvā etannavanītam bhavati svayaivainametattvacā samardhayati tasmādvā abhyaṅkte

He (the sacrificer) then gets himself anointed (with fresh butter), while standing east of the hall. For, having been flayed, man is sore; and by getting himself anointed, he becomes healed of his soreness: for man's skin is on the cow, and that fresh butter also comes from the cow. He (the Adhvaryu) thus supplies him with his own skin, and for this reason he gets himself anointed.