Let a be a non zero integer and p prime.

Show p| => p|a.

This is deeply perplexing me as all I can work out is gcd(p, ) = p. Because p is prime, p has only 1 and p as its divisors. p divides a^2 so p is a divisor of a^2. p>1 so gcd(p,a^2) is p. If it was equal to 1 this would make my job easier but I can't work this out.

any help would be appeciated