Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.4.5

2.6.4.5

sa yadvaiśvadevena yajate | agninaivaitadrājñāgninānīkena caturo māsaḥ prajayati tattryenī śalalī bhavati lohaḥ kṣuraḥ sā yā tryenī śalalī sā trayyai vidyāyai rūpaṃ lohaḥ kṣuro brahmaṇo rūpamagnirhi brahma lohita iva hyagnistasmāllohaḥ kṣuro bhavati tena parivartayate tadbrahmaṇā caivainametattrayyā ca vidyayā parigṛhṇāti

And, accordingly, when he performs the Vaisvadeva, he thereby gains four months, with Agni for his king, with Agni for his leader. Then (in shaving) are used a porcupine's quill spotted in three places, and a copper razor; that three-spotted porcupine's quill resembles the threefold science, and the copper razor resembles the Brahman; for the Brahman is fire, and fire is of reddish (lohita) colour: hence a copper (loha) razor is used. Therewith he has (his head) shaved all round; and thus he (the Adhvaryu) encompasses him with the Brahman and the threefold science.