All perfect praise be to Allah, the Lord of the Worlds; may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Originally, criticism is giving advice and feedback after careful studying. It isn`t only lawful in Islam, but it is also required as it is a form of enjoining kindness and forbidding iniquity. This is because the aim is to guide the criticized person to that which is good and avoid that which is evil. However, this should be done with sincerity since the Messenger of Allah said: "Religion is sincerity, religion is sincerity (Al-Nasihah), religion is sincerity." They said: "To whom, O Messenger of Allah?" He said: "To Allah, to His Book, to His Messenger, to the imams of the Muslims and to their common folk." [Moslim].
Accordingly, every Muslim is required to show sincerity to Allah, His Book, His Messenger, Imams of the Muslims and the common folk. The people of righteousness and fairness like those who criticize them and show them their flaws as this helps them avoid the latter. It was reported that Omar Bin Al-Khattab (May Allah Be Pleased with him) used to say: "May Allah have mercy on the one who had shown me my shortcomings." He showed gratitude to those who had criticized him in kindness because they did him a big favor.
A fair critic must fear Almighty Allah, offer criticism, seeking recompense from Him alone, and perform the duty of enjoining kindness and forbidding iniquity. This is in addition to offering wise criticisms, avoiding thinking ill of the criticized, being meticulous and truthful, avoid cursing, mockery, defamation, injustice, and falsehood since these are considered major sins. In this regard, Messenger of Allah (PBUH) said: "Verily your blood, your property and your honor are as sacred and inviolable as the sanctity of this day of yours, in this month of yours and in this town of yours. Verily! I have conveyed this message to you." [Agreed upon].
Without these controls, criticism opens the door for corrupt individuals to incite enmity, spread rumors, assassinate character, and question achievements under the pretext of criticism. Moreover, criticism is to be delivered by the specialized, not the ordinary people, and facts must be ascertained before any act of criticizing. This is since Almighty Allah Says (What means): "O ye who believe! If a wicked person comes to you with any news, ascertain the truth, lest ye harm people unwittingly and afterwards become full of repentance for what ye have done." [Al-Hujurat/6]. The Prophet (PBUH) said: "It is enough falsehood for a man to relate everything he hears." [Abu Dawood].
It has to be differentiated between criticizing individuals and criticizing their acts affecting society, so it isn`t permissible to transgress against personal life through defamation and slander.
The Prophetic biography, the biography of the companions, the, the followers (At-Tabi`een) and the history of Islam are replete with evidences indicating the permissibility of criticizing. Rather, some pieces of advice were crucial, such as what happened in the Battle of Badr where Al-Habbab Bin Monther advised the Prophet (PBUH) to move the battle to a better location. Since criticism is meant for achieving benefit and reforming the criticized person, it should be directed to the individual in question directly, not defame him or convey that criticism to the public or direct it to that person`s superior who enjoys greater powers.
In conclusion, Islam permits criticism that considers the rules enhancing the value of sincerity in society, separates what is evil from what is good, and achieves common good. And Allah The Almighty Knows Best.