Verse 3.1.2.21

athainaṃ śālām prapādayati | sa dhenvai cānaḍuhaśca nāśnīyāddhenvanaḍuhau vā idaṃ sarvam bibhṛtaste devā abruvandhenvanaḍuhau vā idaṃ sarvam bibhṛto hanta yadanyeṣāṃ vayasāṃ vīryaṃ taddhenvanaḍuhayordadhāmeti sa yadanyeṣāṃ vayasāṃ vīryamāsīttaddhenvanaḍuhayoradadhustasmāddhenuścaivānaḍvāṃśca bhūyiṣṭham bhuṅktastaddhaitatsarvāśyamiva yo dhenvanaḍuhayoraśnīyādantagatiriva taṃ hādbhutamabhijanitorjāyāyai garbhaṃ nirabadhīditi papamakaditi pāpī kīrtistasmāddhenvanaḍuhayornāśnīyāttadu hovāca yājñavalkyo 'śnāmyevāhamaṃsalaṃ cedbhavatīti

Preview of English translation:

21. He (the Adhvaryu) then makes him enter the hall. Let him not eat (the flesh) of either the cow or the ox; for the cow and the ox doubtless support everything here on earth. The gods spake, “Verily, the cow and the ox support everything here: come, let us bestow on the cow and the ox whatever vigour belongs to other species!” Accordingly they bestowed on the cow and the ox whatever vigour belonged to other species (of animals); and therefore the cow and the ox eat most. Hence, were one to eat (the flesh) of an ox or a cow, there would be, as it were, an eating of everything, or, as it were, a going on to the end (or, to destruction). Such a one indeed would be likely to be born (again) as a strange being, (as one of whom there is) evil report, such as 'he has expelled an embryo from a woman,' 'he has committed a sin;' let him therefore not eat (the flesh) of the cow and the ox. Nevertheless Yajnavalkya said, “I, for one, eat it, provided that it is tender.”

For a detailled translation, including proper diacritics and footnotes, go the full English translation.