“How then, after my nirvana, can you eat the flesh of living beings and so pretend to be my disciple? You should know that those who eat meat, though their minds may open and realize a semblance of Samadhi, are but great raksasas who, after this life, will sink back into the bitter ocean of samsara and cannot be my disciples. They will kill and devour one another ceaselessly; how then can they escape from the three worlds of existence?

“In addition you should teach worldly men who practice Samadhi not to kill. This is called the Buddha’s profound teaching of the second decisive deed. Therefore, Ananda, if killing is not stopped, the practice of dhyana-samadhi is like shutting one’s ears while crying in the hope that people will not hear one’s voice, or like trying to hide something that is already exposed to full view. All bhikus who live purely and all Bodhisattvas always refrain even from walking on the grass; how can they agree to uproot it? How then can those who practise great compassion feed on the flesh and blood of living beings? If bhikus do not wear garments made of (Chinese) silk, boots of local leather and furs, and refrain from consuming milk, cream and butter, they will really be liberated from the worldly; after paying their former debts, they will not transmigrate in the three realms of existence. Why? Because by using animal products, one creates causes (which are always followed by effects), just like a man who eats cereals grown in the soil and whose feet cannot leave the ground. If a man can (control) his body and mind and thereby refrains from eating animal flesh and wearing animal products, I say he will really be liberated. This teaching of mine is that of the Buddha whereas any other is that of evil demons.”

— from the, Prohibition Against Killing, Surangama Sutra — a Bilingual English-Vietnamese translation by the Buddha Dharma Education Association — English translation by Charles Luk: https://archive.org/stream/SurangamaSutra-BilangualEnglish-vietnamese/SurangamaSutra_Eng_VN#page/n0/mode/2up/search/shurangama+sutra

Also see (Charles Luk translation): http://www.buddhanet.net/pdf_file/surangama.pdf

From Another Translation

“But, after my nirvana, how will people who eat the flesh of beings deserve to be called disciples of Sakyamuni?

“You should understand that these people who eat flesh may gain some modicum of mental awakening while practising samadhi, but they are all great raksasas who in the end must fall into the sea of death and rebirth. They are not disciples of the Buddha. Such people kill and devour each other, feeding on each other in an endless cycle. How could they possibly get out of the three realms?

“When you teach people in the world to practice samadhi, teach them to renounce all killing. That is the second of the clear and definitive instructions on purity that have been given by the Thus-Come One and by all the Buddhas of the past, World-Honored Ones.

“Therefore, Ananda, one who enters samadhi while practicing meditation in stillness without renouncing all killing is like one who hopes that nobody will hear him shout if he stops up his own ears. He is trying to conceal what is perfectly evident. Bodhisattvas and pure monks walking on country paths will not even tread on living grasses, much less uproot them. How then can it be compassionate to gorge on other beings’ blood and flesh? Monks who will not wear silks from the East, whether coarse or fine; who will not wear shoes or boots of leather, nor furs, nor birds’ down from our own country; and who will not consume milk, curds, or ghee, have truly freed themselves from the world. When they have paid their debts from previous lives, they will roam no longer through the three realms.

“Why? To wear parts of a being’s body is to involve one’s karma with that being, just as people have become bound to this earth by eating vegetables and grains. I can affirm that a person who neither eats the flesh of other beings nor wears any part of the bodies of other beings, nor even thinks of eating or wearing these things, is a person who will gain liberation.

“What I have said is what Buddhas teach. Mara, the Evil One, teaches otherwise.”

— The Surangama Sutra, A New Translation, VII: Four Clear and Definitive Instructions on Purity, On Killing: https://www.scribd.com/document/274447612/Surangama-Sutra-New-Translation

Background on the Surangama Sutra — The Sutra on the Surangama Mantra that is spoken from above the Crown of the Great Buddha’s Head and on the Hidden Basis of the Tathagata’s Myriad Bodhisattva Practices that lead to their Verifications of Ultimate Truth — An original Sanskrit version of Surangama Sutra is not known to be extant, and thus its full Sanskrit name is not known. Surangama roughly means “indestructible.”: https://en.wikipedia.org/wiki/%C5%9A%C5%ABra%E1%B9%85gama_S%C5%ABtra