Arabic-English dictionary, Al-Mawrid, has two meanings that are completely opposite to eachother! Thus jazaa' (جزاء) can either mean reward OR punishment. So :"JazakAllah" can may either mean "may Allah reward you" or "may Allah punish you", while "JazakAllah Khair" means ... "may Allah reward you with the Best / Good" So the correct way is to say Jazakallah khair & not just JazakAllah. Though someone might say that the intention by saying JazakAllah is the same like JazakAllahu khayran, if it's so, then why not use the complete wordings as used by Prophet (ﷺ) and the sahabah. جَزَاکِ اللَّهُ خَيْرًا feminine جَزَاکَ اللَّهُ خَيْرًا masculine جَزَاكُمُ اللَّهُ خَيْرًا plural