بـسـم الله والحـمـد لله والـصلاة والـسـلام عــلى رسـول الله، وبـعـد
The Sunnah is to say جزاك الله خيرا (Jazak Allaahu khair). But we find people saying “Jazak Allaah” only or Jazaks or JAK or JZK– which doesn’t mean anything. So, stick to the Sunnah and avoid falling short of it.
Usaamah bin Zaid رضي الله عنه said: The Messenger of Allaah صلى الله عليه وسلم said:
من صنع إليه معروف ، فقال لفاعله : جزاك الله خيرا ، فقد أبلغ في الثناء
“Whoever has a favor done for him and says Jazaak Allaahu khayran (Allaah reward you with good) has done his utmost to thank him.”
[Narrated by al-Tirmidhee (2035) and graded as “Saheeh” by Shaikh al-Albaanee in Saheeh al-Tirmidhee]