What is the ruling on saying, "May Allah reward you with a thousand good deeds"?

It is sunnah to supplicate for someone who has done a favor for you, especially if you do not find anything to reward them with, such as saying: "May Allah reward you with good (Jazakallahu khairan)"; based on the hadith: "Whoever has a favor done for him and says to the doer of it: 'May Allah reward you with good (Jazakallahu khairan),' he has indeed praised him sufficiently." There is no harm in saying: "May Allah reward you with a thousand good things (Jazakallahu alf khair)," although it is better to adhere to what was authentically reported from the Prophet, peace and blessings be upon him.