Where is definition of getAuthPassword of user Model in laravel 5.1?

I am right now in File :

\vendor\laravel\framework\src\Illuminate\Auth\EloquentUserProvider.php

Function : validateCredentials

There is below mentioned code

return $this->hasher->check($plain, $user->getAuthPassword());

I am trying to find out why the value of $user->getAuthPassword() is null and that's the reason I want to know the location for definition of this function.

via Chebli Mohamed