Laravel 4 custom validation rule - where to extend the validator?

I want to make a custom validation rule. My model looks like this at the moment:

protected $rules = array(
    'first_name'  => 'required',
    'last_name'   => 'required',
    'ssn'         => 'required|integer|min:4|max:4',
    'email'       => 'required|email',
    'dob'         => 'required|checkAge',
    'phone'       => 'required',
    'street'      => 'required',
    'postal_code' => 'required|integer|min:4',
    'city'        => 'required'
);

But where I have to put the custom validation rule? I have read that I need to extend it Validator. For this I've created a simple function

Validator::extend('foo', function($attribute, $value, $parameters)
{
    return $value == 'foo';
});

And I don't know where I have to check it?

Maybe someone can help me.

Thanks