Usaamah ibn Zaid narrates that the Messenger of Allah (ﷺ) said:
مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ فَقَالَ لِفَاعِلِهِ جَزَاكَ اللَّهُ خَيْرًا فَقَدْ أَبْلَغَ فِي الثَّنَاءِ
“Whoever some good was done to him, and he says, ‘ jazaak Allaahu khairan (May Allah reward you with goodness)’ then he has done the most that he can do of commendation.”
[al-Tirmidhi #2035, hasan]
Imam al-Mubarakfoori wrote in explanation of this hadith:
ـ (جزاك الله خيراً) أي خير الجزاء أو أعطاك خيراً من خيري الدنيا والاَخرة (فقد أبلغ في الثناء) أي بالغ في أداء شكره وذلك أنه اعترف بالتقصير وأنه ممن عجز عن جزائه وثنائه ففوض جزاءه إلى الله ليجزيه الجزاء الأوفى. قال بعضهم: إذا قصرت يداك بالمكافأة، فليطل لسانك بالشكر والدعاء.
“Jazaak Allaahu khairan (May Allah reward you with goodness)” – meaning a good reward, or to give you khair in the dunya and the aakhira. “Then he has done the most that he can do of commendation” – meaning he has accomplished the manifestation of his gratitude, and that is that he realizes his inability, and that he is incapable to reward and praise his benefactor sufficiently, so he entrusts his benefactor’s reward to Allah to recompense him with a reward and full repayment. And some have said, “when you are unable with your hands to make an adequate remuneration, then keep your tongue moist with gratitude and du’a.”
[Tuhfat al-Ahwadhi #2035]