Where is the reference mentioned in the Wikipedia article for the General Chernoff inequality? I cannot find it.
If you mean that there is no direct link to a reference in the section of the entry you linked, you're correct. There are no links to any relevant references provided that support or pertain to that section, in that particular entry.
However, there is a link, in the given entry, to another Wikipedia entry: the Chernoff Bound (a separate entry), where the Chernoff inequality/bound is discussed and developed in greater detail, and many references are provided in this entry (one reference is even a paper authored by Chernoff).
Indeed, when I used the expression "Chernoff Inequality" in the search field of Wikipedia, it took me directly to the entry whose link I've provided.
Added: If you're looking for a particular form of the inequality, try googling "Chernoff's Inequality"...when I did so, I came up with a lot of "hits": lecture notes, discussions, etc., which may provide you with more information and/or references. P.S. Is this the form of the inequality in which you're interested in? It's from your linked entry.
If you are referring to a different formulation, please include that in your question. I guess I'm not clear as to the inequality (that is, the specific form, or variation) you're using. Perhaps adding to your question some context as to what it is you need it for/and why in this form only, its significance to your work etc?
Do keep us posted if you have any further clarification of your question, and/or if you find what you are looking for! Good luck.