Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.2.1

3.1.2.1

aparāhṇe dīkṣeta | purā keśaśmaśrorvapanādyatkāmayeta tadaśnīyādyadvā sampadyeta vrataṃ hyevāsyāto 'śanam bhavati yadyu nāśiśiṣedapi kāmaṃ nāśnīyāt

Let him perform the rite of consecration (dîkshâ) in the afternoon. Previously to the shaving of his hair and beard he may eat of what he likes, or whatever comes to hand; for hereafter his food consists of fast-milk (vrata) only. But, if he does not care to eat, he need not eat anything.