Ruling on saying: Jazakum Allaahu alfa khayrin (May Allaah reward you with good one thousand times)
Question

                What is the ruling on saying Jazakum Allaahu alfa khayrin (May Allaah reward you with good one thousand times)? Is it a phrase that is not allowed?.
Praise be to Allah.The Prophet  (peace and 
    blessings of Allaah be upon him) taught us to rewards the one who does us a 
    favour, and if we cannot do that then we should pray to Allaah  for him and 
    say: Jazaak Allaahu khayran (may Allaah reward you with good). 
Abu Dawood (1672) narrated 
    that ‘Abd-Allaah ibn ‘Umar (may Allaah be pleased with him) said: The 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) said: 
    “Whoever does you a favour, then reciprocate, and if you cannot find 
    anything with which to reciprocate, then pray for him until you think that 
    you have reciprocated him.”  Classed as saheeh by al-Albaani in Irwa’ 
    al-Ghaleel (1617).   
Al-Tirmidhi (2035) narrated 
    that Usaamah ibn Zayd (may Allaah be pleased with him) said: The Messenger 
    of Allaah  (peace and blessings of Allaah be upon him) said: “Whoever 
    has a favour done for him and says to the one who did it, ‘Jazak Allaahu 
    khayran,’ has done enough to thank him.”  Classed as saheeh by al-Albaani in
    Saheeh al-Tirmidhi. 
“has done enough to thank 
    him” means: he has thanked him sufficiently, by acknowledging his 
    shortcoming and his inability to reward him, so he defers his reward to 
    Allaah so that He might reward him abundantly. 
This is the wording that 
    was narrated from the Prophet  (peace and blessings of Allaah be upon 
    him), “Jazak Allaahu khayran (may Allaah reward you with good)”, and this is 
    undoubtedly more perfect than any other. The best of guidance is the 
    guidance of the Prophet  (peace and blessings of Allaah be upon him) so 
    we should not ignore this and use something else instead. 
If the Muslim says Jazakum 
    Allaahu alfa khayrin (May Allaah reward you with good one thousand times) 
    occasionally, there is nothing wrong with it, so long as he accepts that the 
    wording of the Prophet  (peace and blessings of Allaah be upon him) is 
    more perfect and better. 
And Allaah knows best.