The Sunnah is to say جزاك الله خيرا (Jazak Allaahu khair). But we find people saying “Jazak Allaah†only or Jazaks or JZK – which doesn’t mean anything. So, stick to the Sunnah and avoid falling short of it.
The Messenger of Allaah صلى الله عليه وسلم said: “Whoever has a favor done for him and says Jazaak Allaahu khayran (Allaah reward you with good) has done his utmost to thank him.â€
[Narrated by al-Tirmidhi, 2035 classed as saheeh by al-Albaani in Saheeh al-Tirmidhi]