The earliest I can find is Ignatius. That would be one generation from the NT church.
Ignatius speaks of this sacrament in two passages, only by way of allusion, but in very strong, mystical terms, calling it the flesh of our crucified and risen Lord Jesus Christ, and the consecrated bread a medicine of immortality and an antidote of spiritual death. Schaff, P., & Schaff, D. S. (1910). Vol. 2: History of the Christian church (241). New York: Charles Scribner’s Sons.
With the early battles against Gnostic Doceticism, it appears Ignatius and others overstated the "flesh."
My set on "The History of the Holy Eucharist" tends to place it with Justin Martyr, Tertullian and Origen.