Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.4.7

2.6.4.7

atha yatsākamedhairyajate | indreṇaivaitadrājñendreṇānīkenāparāṃścaturo māsaḥ prajayati tattryenī śalalī bhavati lohaḥ kṣurastena parivartayate tadbrahmaṇā caivainametattrayyā ca vidyayā parigṛhṇāti

And when he performs the Sâkamedha offerings, he thereby gains other four months, with Indra for his king, with Indra for his leader. Then a three-spotted quill of a porcupine and a copper razor are used, wherewith he has himself shaved; and thus he (the priest) encompasses him with the Brahman and the threefold science.