Shatapatha Brahmana 11.5.4.12

11.5.4.12

tadapi ślokaṃ gāyanti ācāryo garbhī bhavati hastamādhāya dakṣiṇam tṛtīyasyāṃ sa jāyate sāvitryā saha brāhmaṇa iti sadyo ha tvāva brāhmaṇāyānubrūyādāgneyo vai brāhmaṇaḥ sadyo vā agnirjāyate tasmātsadya eva brāhmaṇāyānubrūyāt

Concerning this they also sing the verse,--'By laying his right hand on (the pupil), the teacher becomes pregnant (with him): in the third (night) he is born as a Brâhmana with the Sâvitrî.' Let him, however, teach a Brâhmana (the Sâvitrî) at once, for the Brâhmana belongs to Agni, and Agni is born at once: therefore, he should teach the Brâhmana at once.