I am looking for an exact reference (something I can site in
bibliography) of Aristotle's axiom, "Nature abhors a vacuum."  Thanks

Based on my preliminary research, I doubt very much that there is a
single citable source for the origin of the exact phrase "nature
abhors a vacuum" or its Greek equivalent.  Although some attribute it
to Aristotle (and some to Spinoza), the phrase is usually considered
to be a proverb, and thus unattributable by definition.

If a citation to Aristotle's discussion of vacuums, which is
consistent with the proverb, but does not contain it, would suit your
purposes, let me know.

"'Natura abhorret vacuum' is an expression found in numerous post classical
writers":
http://omega.cohums.ohio-state.edu/mailing_lists/CLA-L/Older/log94/9408/9408.225.html

In case you understand Italian:
http://www.imss.firenze.it/vuoto/iprota.html

"The Battle of Void":
http://inicia.es/de/aribas/esquemae.html