This motet possibly quotes pre-existing material, but it can not be linked to any Cantus prius factus with a corresponding text.
See http://www.kalosconcentus.org/index.php/f-gaffurio/ave-verum-corpus.
See the notes by Sergio Lonoce at http://www.kalosconcentus.org/index.php/f-gaffurio/ave-verum-corpus.