laravel/app/Http/Filters/MaintenanceFilter.php

<?php namespace App\Http\Filters;

use Illuminate\Http\Response;
use Illuminate\Contracts\Foundation\Application;

class MaintenanceFilter {

	/**
	 * The application implementation.
	 *
	 * @var Application
	 */
	protected $app;

	/**
	 * Create a new filter instance.
	 *
	 * @param  Application  $app
	 * @return void
	 */
	public function __construct(Application $app)
	{
		$this->app = $app;
	}

	/**
	 * Run the request filter.
	 *
	 * @return mixed
	 */
	public function filter()
	{
		if ($this->app->isDownForMaintenance())
		{
			return new Response('Be right back!', 503);
		}
	}

}