Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.1.24

7.4.1.24

te cetayamānāḥ | etatsāmāpaśyaṃstadagāyaṃstadasminvīryamadadhustathaivāsminnayametaddadhāti puruṣe gāyati puruṣe tadvīryaṃ dadhāti citre gāyati sarvāṇi hi citrāṇyagnistamupadhāya na purastātparīyānnenmāyamagnirhinasaditi

Whilst meditating, they saw this Sâman, and sang it, and thereby put vigour into him; and in like manner does this (Sacrificer) thereby put it into him: he sings on the man, he puts vigour into the man;--he sings on the bright one, for Agni is all bright things. After he has laid him down, let him not walk round him in front, lest that Agni should injure him.