Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.2.10

3.2.2.10

athāsmai vrataṃ śrapayanti | devānvā eṣa upāvartate yo dīkṣate sa devatānāmeko bhavati śṛtaṃ vai devānāṃ havirnāśṛtaṃ tasmācrapayanti tadeṣa eva vratayati nāgnau juhoti tadyadeṣa eva vratayati nāgnau juhoti

Thereupon they cook the fast-food for him. For he who is consecrated draws nigh to the gods and becomes one of the deities. But the sacrificial food of the gods must be cooked, and not uncooked: hence they cook it, and he partakes of that fast-milk (vrata) and does not offer it in the fire. The reason why he eats the fast-food and does not offer it in the fire is this: