You can read it here The poem gets shared a lot in vegan circles but I can't find where it was originally published.
The earliest appearance of this poem that I was able to locate is in Religion & Peace (1957) by S. C. Diwaker:
In this regard the words of George Bernard Shaw are ever memorable:
We are the living graves of murdered beasts,
Slaughtered to satisfy our appetites
We never pause to wonder at our feasts
If kine, like men, can possibly have rights.
S. C. Diwaker (1957). Religion & Peace, p. 209. Jabalpur: All India Digamber Jain Sangh Mathura.
The sentence introducing the poem is ambiguous: clearly some people have interpreted it as saying that the poem is by Shaw himself, but another possibility is that it means that Diwaker was inspired to write this poem by the words of Shaw.
The reasons that I prefer the second interpretation are (i) I couldn’t find any evidence that the poem, or anything resembling it, appeared in print prior to 1957; (ii) the poem is rather amateurish, and although Shaw was not a poet† I find it hard to imagine him being satisfied, for example, with the line “We pray for it, o’er the tombs of slain” which fails to scan, or the pleonasm of “guide our footsteps on the path we tread”.
† There is a book purporting to contain Eighteen Love Poems for Ellen Terry Attributed to George Bernard Shaw (ed. Jack Werner, 1980), but “if you believe that the dusty painting in your attic is a Rembrandt, or if you’re sure that the chipped old fiddle in your basement is a Stradivarius, then you might even believe that these poems were written by Bernard Shaw for Ellen Terry. The less credulous will note that the poems are written in a suffocatingly sentimental and humorless stvle that is completely unlike Shaw’s” (Shaw: the Journal of Bernard Shaw Studies, vol. 2, p. 200, 1982).
But if ‘Living Grave’ was written by Diwaker, what are the “words of Shaw” that were so memorable? I suspect that Diwaker had encountered the following passage by Isadora Duncan:
Bernard Shaw says that as long as men torture and slay animals and eat their flesh, we shall have war. I think all sane, thinking people must be of his opinion. The children of my School were all vegetarians and grew strong and beautiful on a vegetable and fruit diet. Sometimes during the war when I heard the cries of the wounded I thought of the cries of the animals in the slaughter house, and I felt that as we torture these poor defenceless creatures so the gods torture us. Who loves this horrible thing called War? Probably the meat eaters, having killed, feel the need to kill—kill birds, animals—the tender stricken deer—hunt foxes.
The butcher with his bloody apron incites bloodshed, murder. Why not? From cutting the throat of a young calf to cutting the throat of our brothers and sisters is but a step. While we are ourselves the living graves of murdered animals, how can we expect any ideal conditions on the earth?
Isadora Duncan (1927). My Life, p. 309. New York: Garden City Publishing.
The highlighted phrase is very close to the first line of the poem, and in the context of “Bernard Shaw says” a reader may have acquired the impression that these were Shaw’s own words, rather than Duncan’s. The removal of “while” and “ourselves”, and the substitution of “beasts” for “animals” were necessary changes to make the phrase into a line of iambic pentameter.