1. It is permissible to compliment people for the good they do as this is a means of encouragement for them and a way of showing appreciation.
عن أبي بن كعب، قال: قال رسول الله صلى الله عليه وسلم: يا أبا المنذر، أتدري أي آية من كتاب الله معك أعظم؟ قال: قلت: الله ورسوله أعلم. قال: يا أبا المنذر أتدري أي آية من كتاب الله معك أعظم؟ قال: قلت: {الله لا إله إلا هو الحي القيوم}. قال: فضرب في صدري، وقال: والله ليهنك العلم أبا المنذر (صحيح مسلم، الرقم: 810)
Hazrat Ubayy (radhiyallahu ‘anhu) reports that on one occasion Rasulullah (sallallahu ‘alaihi wasallam) posed the following question to him, “O Abul Mundhir, do you know which is the greatest verse in the book of Allah Ta’ala with you?” Hazrat Ubayy (radhiyallahu ‘anhu) replied, “Allah Ta’ala and His messenger know best.” Rasulullah (sallallahu ‘alaihi wasallam) asked again, “O Abul Mundhir, do you know which is the greatest verse in the book of Allah Ta’ala with you?” Hazrat Ubayy (radhiyallahu ‘anhu) then replied, “The greatest verse is Aayatul Kursi.” Rasulullah (sallallahu ‘alaihi wasallam) expressed his approval by hitting my chest and saying, “O Abul Mundhir, May Allah Ta’ala bless you in your knowledge.”