First, mistakes were made:
-I slightly misquoted the sentence
-The author is Guatemalan, not Spanish, and
-I think I may have said the essay was by Umberto Eco instead of Italo Calvino, which was totally my bad (can’t keep my high-concept Italian authors straight), but at any rate: the sentence was quoted in Six Memos for the Next Millennium by Italo Calvino, in the essay called “Quickness”:
“Borges and Bioy Casares put together an anthology of short extraordinary tales (Cuentos breves y extraordinarios, 1955). I would like to edit a collection of tales consisting of one sentence only, or even a single line. But so far I haven’t found any to match the one by the Guatemalan writer Augusto Monterroso: ‘Cuando desperto, el dinosauro todavia estaba alli’ (When I woke up, the dinosaur was still there).”