no one can testify the purity of others against Allah
Abu Bakra, may Allah be pleased with him, narrated:
"A man praised another man in front of the Prophet. The Prophet (may the peace and blessings of Allah be upon him) said thrice, "Wailaka (Woe on you) ! You have cut the neck of your brother!" The Prophet (may the peace and blessings of Allah be upon him) added, "If it is indispensable for anyone of you to praise a person, then he should say, "I think that such-and-such person (is so-and-so), and Allah is the one who will take his accounts (as he knows his reality) and none can sanctify anybody before Allah (and that only if he knows well about that person.)".
This Hadith teaches us that we can not confirm the end of anyone or even his good intention because all of these matters are part of the unseen.
The well mannered man properly estimates the rights of his brother, he may commend him duly. Undoubtedly the praise has a good impact on the souls. Moreover, it encourages doing more good.
People are of different types in terms of the effect of the commendation;
Some of them may be arrogant, proud and continue to show off.
Some of them will be too lazy to realize the high pursuits for he is sufficed with the praise of the people so he aspires nothing more than it. This is because he reached the rank of praise.
Another type of people will feel shy if praised, and be severely embarrassed.
However, the best type of people are those who increase in faith if people praised them. They will be keen to do more good deeds, and work hard to meet the good expectations of people.
Yet, there is a type of praise that is considered a matter of deception in order to obtain a worldly gain, this is type is dispraised according to the Sharia law.
The Prophet (may the peace and blessings of Allah be upon him) praised many of his companions and gave them the glad tidings of entering the Paradise for their faith and devotion to Allah both in words and deeds.
The believer is sincere in tone, strong in his argument, he does not act like the hypocrites who meet a group of people with a face and then change into another face when meeting other people.