Śatapatha Brāhmaṇa ("of a hundred paths") 11.3.3.7

11.3.3.7

na ha vai snātvā bhikṣeta apa ha vai snātvā bhikṣāṃ jayatyapa jñātīnāmaśanāyāmapa pitṝṇāṃ sa evaṃ vidvānyasyā eva bhūyiṣṭhaṃ ślāgheta tām bhikṣetetyāhustallokyamiti sa yadyanyāṃ bhikṣitavyāṃ na vindedapi svāmevācāryajāyām bhikṣetātho svām mātaraṃ nainaṃ saptamyabhikṣitātīyāttamevaṃ vidvāṃsamevaṃ carantaṃ sarve vedā āviśanti yathā ha vā agniḥ samiddho rocata evaṃ ha vai sa snātvā rocate ya evaṃ vidvānbrahmacaryaṃ carati

Let him not beg alms after he has bathed (at the end of his studentship), for by bathing he drives off beggary, and drives off hunger from his kinsmen and his deceased ancestors, 'Let him who knows this beg alms only from her in whom he has the greatest confidence,' they say, 'for that makes for heaven.' And should he find no other woman from whom alms could be begged, he may even beg from his own teacher's wife, and thereafter from his own mother. The seventh (night) should not pass by for him without begging: him who knows this and practises this all the Vedas enter; for, verily, even as the fire shines when kindled, so does he, after bathing, shine, who, knowing this, lives a Brahmakârin's life.