Specialization of template function after point of use will break the compilation

Question

Consider next example :

#include <iostream>

template< int a >
void foo();

int main(int argn, char* argv[])
{
    foo<1>();
}

template<>
void foo<1>()
{
    std::cout<<1<<std::endl;
}

The compilation fails with next error messages :

rg.cpp:12: error: specialization of ‘void foo() [with int a = 1]’ after instantiation

What paragraph in the standard explains this error?

PS :I know that if I move the function definition in front of main will make the error go away.