"...and because of this, the Prophet exposed for us those who please us with their tongues, when he said: "Throw dirt in their faces," meaning, the faces of those who praise people to their faces. Likewise, he said to the one who praised his brother to his face: "You have broken your brother's back." So, praising your brother to his face is disliked - if not forbidden - except in the case that you are trying to point out some of his faults. In such a situation, there is no problem in you mentioning some of his praiseworthy traits, such as saying to him: 'Brother, you are very intelligent, and you are a man who is loved by the people, and you are a person who is considered a leader. However, I see in you such-and-such a fault, so, is it possible for you to correct these faults?' And if the person that you wish to advise has a position of authority over you, is older than you, or is your parent, then there is no problem in sending him a message, for example.
Hasan al-Banna - may Allah have Mercy upon him - said: "We used to follow this way of commanding the good and forbidding the evil. We had a shaykh who used to teach us and guide us, so, one day, I saw him praying between the pillars of the mosque. So, I wanted to advise him that praying between the pillars of the mosque is disliked (makruh). So, I wrote him a letter, and addressed it on behalf of 'A Doer of Good,' saying: "O Shaykh, I saw you praying between the pillars of the mosque, and this is disliked, as the Prophet said." I then signed it 'A Doer of Good,' and sent it to him by mail. He got the letter and read it, then he said: "O youth! I have received a letter from a man who advised me not to pray between the pillars of the mosque, and I did not previously know that this is disliked, so, don't do it.""
Hasan al-Banna added: "I was amongst the youth that he was addressing. So, we were able to fulfill a good action without causing any insult or offense to our teacher...."
Therefore, commanding the good and forbidding the evil requires a person who loves the people; a person who looks at the bigger picture; a person with a gentle tongue. Do not come to one and say: 'I hate you for the Sake of Allah because you do this or that.' Are you not able to say, instead: 'I love you for the Sake of Allah, my brother! However, I saw a simple and small mistake from you.'
By Allah, a brother described to me the following: "Someone came up to me and said: "I hate you for the Sake of Allah."
So, I said to him: "Why? Why do you hate me for the Sake of Allah?"
He replied: "Because your father is from the Ikhwan al-Muslimin.""
There is no might nor power except with Allah. What Islam is this? I hate him for the Sake of Allah - for what? Because his father is from the Ikhwan al-Muslimin. Sufficient is Allah as the Disposer of our affairs, and this person considers this to be commanding the good and forbidding the evil and a proclaimation of the truth, and he thinks that he will have some reward as a result of this, as a result of his putting off the Muslims..."
['Fi Dhilal Surat at-Tawbah'; p. 75]
(SHEIKH 'ABDULLAH YUSUF 'AZZAM (ASY-SYAHID, INSYAALLAH)