CC Ādi 17.159

prabhu kahe, — vede kahe go-vadha niṣedha
ataeva hindu-mātra nā kare go-vadha
Synonyms: 
prabhu kahe — the Lord replied; vede — in the Vedas; kahe — is enjoined; go-vadha — cow-killing; niṣedha — prohibition; ataeva — therefore; hindu — Hindu; mātra — any; — does not; kare — execute; go-vadha — cow-killing.
Translation: 
Refuting the Kazi’s statement, the Lord immediately replied, “The Vedas clearly enjoin that cows should not be killed. Therefore every Hindu, whoever he may be, avoids indulging in cow-killing.
Purport: 

In the Vedic scriptures there are concessions for meat-eaters. It is said that if one wants to eat meat, he should kill a goat before the goddess Kālī and then eat its meat. Meat-eaters are not allowed to purchase meat or flesh from a market or slaughterhouse. There are no sanctions for maintaining regular slaughterhouses to satisfy the tongues of meat-eaters. As far as cow-killing is concerned, it is completely forbidden. Since the cow is considered a mother, how could the Vedas allow cow-killing? Śrī Caitanya Mahāprabhu pointed out that the Kazi’s statement was faulty. In the Bhagavad-gītā (18.44) there is a clear injunction that cows should be protected: kṛṣi-gorakṣya-vāṇijyaṁ vaiśya-karma svabhāva-jam. “The duty of vaiśyas is to produce agricultural products, trade and give protection to cows.” Therefore it is a false statement that the Vedic scriptures contain injunctions permitting cow-killing.