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.
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