routes.php

<?php

use App\User;

Route::group(['prefix' => 'user'], function () {
    Route::fallback(function () {
        return response()->view('custom-error', [
            'message' => 'Usuário deve conter apenas caracteres numéricos.',
        ])
            ->setStatusCode(404);
    });

    Route::get('{id}', function ($id) {
        return User::find($id);
    })->where('id', '[0-9]+');
});