UserController.php

<?php

namespace App\Http\Controllers\Admin;

use App\User;
use Illuminate\Http\Request;
use App\Http\Controllers\Controller;
use Illuminate\Support\Facades\Auth;
use Illuminate\Support\Facades\Validator;

class UserController extends Controller
{
    /**
     * @param Request $request
     * @return \Illuminate\Http\RedirectResponse|\Illuminate\Routing\Redirector
     */
    public function login(Request $request)
    {
        $rules = [
            'email' => 'required|email|exists:users,email',
            'password' => 'required',
        ];

        $validator = Validator::make($request->all(), $rules);

        /** @var User $user */
        $user = Auth::attempt([
            'email' => $request->email,
            'password' => $request->password
        ]);

        $validator->after(function ($validator) use ($user) {
            if (!$user) {
                $validator->errors()->add('field', 'Something is wrong with this field!');
            }
        });

        if ($validator->fails()) {
            return redirect('/admin')
                ->withErrors($validator);
        } else {
            if ($user->isAdmin()) {
                return redirect()
                    ->route('admin.dashboard');
            }
        }

        return redirect('/admin/login');
    }
}