Magento 2

vendor/magento/module-checkout/Controller/Onepage # vi Success.php +26

comment out:

//        $session->clearQuote();

Magento 2

vendor/magento/module-checkout/Controller/Onepage/Success.php

comment out below line :

//        $session->clearQuote();

Please note that, as the current session quote is already cleared, You need to place one order again after commenting out the above line.