It's an obligation for every Muslim to give each other advice (naseehah). This is because every Muslim is a brother and it would be his brother who had to reminded him/her if him/her do wrong or sin.
How many of us have looked to our brothers and sisters in Islam and have seen them straying from the path of Allah, and have turned our faces away? How many of us have seen our brothers erring and said to ourselves, it is none of my business? Well, it is our business for the Prophet (sallAllahu 'alaihi wa sallam) has made it our business. In the hadith related by Tamim ad-Dari, the Prophet (sallAllahu 'alaihi wa sallam) said, "The religion is naseehah." The people asked, "To whom?" The Prophet (sallAllahu 'alaihi wa sallam) replied, "To Allah and to His Book and to His Messenger and to the leaders of the Muslims and the common folk." [Collected by Bukhari, Muslim]
Giving advice to other people need some approach appart. Below are some manners, which must be observed by a Muslim when giving advice (naseehah) to his brother/sister:
1. Seeking the Pleasure of Allah by giving Naseehah
It is necessary that a person has the intention of seeking the pleasure of Allah when giving naseehah. Only such an intention deserves reward from Allah and acceptance from His slaves.
If the intention is other than that, then that person deserves the anger and wrath of Allah as well as the hatred and rejection of the people - including the one being advised.
2. Not slandering the one being advised
This is an affliction that has befallen many Muslims. Many times, after taking a closer look, we find that the person giving naseehah actually wants to slander the person he is advising because of personal hatred. This does not befit the one being advised and may lead to a worse situation with no benefit resulting from the naseehah.
3. Naseehah is to be given in secret
Naseehah is most likely to bear its fruit when given to a person when he is by himself, for in such a situation the person is less likely to be affected by the thoughts of others. The sincere advisor should not aid the Devil over his brother by publicly rebuking him and letting Shaitaan beguile his brother into not taking the naseehah. This closes the doors of goodness and acceptance, and reduces the chances of the naseehah from being accepted.
Fudail Ibn Ayyadh, one of the pious scholars from our predecessors, said, "A believer covers up and gives naseehah, whereas an evildoer exposes and humiliates." Ibn Rajab commented on Fudail's saying, "It is naseehah if it is with a cover, while humiliating is with broadcasting."
4. Naseehah is to be given with kindness, gentleness and softness
A sincere advisor must be kind, soft and well-mannered in giving naseehah to others, as this might get the desired response from the one he is advising. One must understand that accepting naseehah is like opening a door, and that the door will not open without the proper key. The one who is given naseehah has a heart that has a lock in some matter - for he has abandoned something that Allah has demanded from him, or has committed something that Allah had forbidden him from.
5. Do not compel others to follow one's Naseehah
It is wajib (obligatory) on the advisor to render sincere advice to others, but it is not his right to compel others to follow his advice as well. That is the right of the Muslim ruler upon his subjects or a Muslim Qadhi (Judge) in his jurisdiction. A sincere advisor is one who guides toward goodness, but he is not to command others to act upon it.
6. Choosing the proper time to give Naseehah
The one giving naseehah must choose the right time to give his advice, since a person is not always ready to receive naseehah.A person may be angry about something, upset about not getting what he wanted, grieved for something he may have lost, or there may be some other reason that might prevent him from responding to the naseehah.
7. Naseehah that is against Islam is not to be followed
Giving naseehah is part of Islamic Sharia. Therefore, if someone gives advice to leave a deen demanded by the Sharia or to perform a forbidden deed, then it is not called naseehah. The one giving such should quit doing that and the one being advised should not accept it.