Perfection in showing gratitude to someone is saying "Jazāk Allāhu Khayran"

the Messenger of Allah ﷺ said, "Whoever some good was done to him, and he says: 'May Allah reward you in goodness' then he has done the most that he can of praise."  

قَالَ رَسُولُ اللَّهِ ﷺ «مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ فَقَالَ لِفَاعِلِهِ جَزَاكَ اللَّهُ خَيْرًا فَقَدْ أَبْلَغَ فِي الثَّنَاءِ۔»  

[Caution: Unverified AI Translation] We were with Ibn Jurayj, the Makki, when a questioner came and asked him. Ibn Jurayj then said to his treasurer, "Give him a dinar." The treasurer said, "I only have one dinar, if I give it to him, you will be hungry, and so will your family." Ibn Jurayj became angry and said, "Give it to him." Al-Makki said, "We were with Ibn Jurayj when a man came to him with a letter and a purse. Some of his brothers had sent it to him, and in the letter, it said, 'I have sent fifty dinars.' Ibn Jurayj opened the purse and counted it, and behold, there were fifty-one dinars. Ibn Jurayj said to his treasurer, 'You have given one, so may Allah replace it for you and increase you by fifty dinars.'"  

كُنَّا عِنْدَ ابْنِ جُرَيْجٍ الْمَكِّيِّ فَجَاءَ سَائِلٌ فَسَأَلَهُ فَقَالَ ابْنُ جُرَيْجٍ لِخَازِنِهِ أَعْطِهِ دِينَارًا فَقَالَ مَا عِنْدِي إِلاَّ دِينَارٌ إِنْ أَعْطَيْتُهُ لَجُعْتَ وَعِيَالَكَ قَالَ فَغَضِبَ وَقَالَ أَعْطِهِ قَالَ الْمَكِّيُّ فَنَحْنُ عِنْدَ ابْنِ جُرَيْجٍ إِذْ جَاءَهُ رَجُلٌ بِكِتَابٍ وَصُرَّةٍ وَقَدْ بَعَثَ إِلَيْهِ بَعْضُ إِخْوَانِهِ وَفِي الْكِتَابِ إِنِّي قَدْ بَعَثْتُ خَمْسِينَ دِينَارًا قَالَ فَحَلَّ ابْنُ جُرَيْجٍ الصُّرَّةَ فَعَدَّهَا فَإِذَا هِيَ أَحَدٌ وَخَمْسُونَ دِينَارًا قَالَ فَقَالَ ابْنُ جُرَيْجٍ لِخَازِنِهِ قَدْ أَعْطَيْتَ وَاحِدًا فَرَدَّهُ اللَّهُ عَلَيْكَ وَزَادَكَ خَمْسِينَ دِينَارًا