Neither Waring or Wilson could prove the above theorem, but now it can
be found in any elementary number theory text. To save you some time we
present a proof here.
It is easy to check the result when p is 2 or 3, so let us
assume p > 3.
If p is composite, then its positive divisors
are among the integers
1, 2, 3, 4, ... , p-1
and it is clear that gcd((p-1)!,p) > 1, so we can not
have (p-1)! = -1 (mod p).
However if p is prime, then each of the above
integers are relatively prime to p. So for each of these integers
a there is another b such that ab = 1
(mod p). It is important to note that this b is unique
modulo p, and that since p is prime, a = b
if and only if a is 1 or p-1. Now if we omit 1 and
p-1, then the others can be grouped into pairs whose
product is one showing
= 1 (mod p)
(or more simply (p-2)! = 1 (mod p)). Finally, multiply
this equality by p-1 to complete the proof.