This seems to be the original source:

M. Barr and J. Beck, <A HREF="https://www.math.mcgill.ca/barr/papers/amt.pdf">Acyclic models and triples,</A> Proc. Conference Categorical Algebra (La Jolla, Calif., 1965) Springer, New York, 1966, pp. 336-343.

The "triple" is an older name for a monad.

Also, Jonathan Mock Beck is the same as Jon Beck, as you can see from these [lecture notes](http://www.tac.mta.ca/tac/reprints/articles/18/tr18.pdf) and this [Wikipedia page](https://en.wikipedia.org/wiki/Jonathan_Mock_Beck).