source: app/Http/Requests/Dashboard/UsernameSettingsRequest.php

<?php

namespace App\Http\Requests\Dashboard;

use Illuminate\Foundation\Http\FormRequest;
use Illuminate\Contracts\Validation\Validator;
use Illuminate\Validation\ValidationException;

class UsernameSettingsRequest extends FormRequest
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        return [
            "username" => "required|alpha_dash|min:5|unique:users,username,$this->id,id"
        ];
    }

    protected function failedValidation(Validator $validator)
    {
        $response = redirect()
            ->route('dashboard.settings.index')
            ->withInput(['active_tab' => 'security'])
            ->withErrors($validator);

        throw (new ValidationException($validator))
            ->errorBag($this->errorBag)
            ->redirectTo($this->getRedirectUrl());
    }
}