Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.1.31

2.3.1.31

tadu haitadevāruṇaye brahmavarcasakāmāya takṣānūvācāgnirvarco jyotirvarcaḥ sūryo varco jyotirvarca iti brahmavarcasī haiva bhavati ya evaṃ vidvānagni hotraṃ juhoti

Here now Takshan recited for Âruni, who wished to obtain holy lustre (brahmavarkasa, inspired nature), 'Agni is lustre, light is lustre;'--'Sûrya is lustre, light is lustre.' Holy lustre, therefore, he obtains whosoever, knowing this, thus offers the Agnihotra.