One piece of the Sharia evidence in support of freedom of speech may be found under the principle of commanding good and forbidding evil (al-amr bil ma‘ru̅f wa an-nahi ‘an al-munkar, aka ḥisbah) as it guarantees individuals the basic freedom of to formulate and express their own opinions in certain contexts. According to this principle, individuals, pursuant to their status and capacities, are entitled to speak or to act in pursuit of what in their enlightened discretion seems good and forbid, whether by words, actions or even in silent denunciation, an act that they perceive as an evil being committed.
Linguistically, ma‘ru̅f (opposite munkar) means anything that one knows within one's self to be good. In the technical sense, it is a general term that includes all acts of obedience to God and acts of benevolence to people. Munkar, on the other hand, is linguistically an antonym of ma‘ru̅f, and is technically defined as anything that is known to be abominable by virtue of intellect and as per religious legislation. Ḥisbah is so highly recognized and established in Islam that Al-Ghaza̅li considers it "the greatest pole in religion" (al-quṭb al-a‘ẓam fi ad-di̅n), and the most important objective of all of God's revealed scriptures. He also reckons that a total disregard of ḥisbah, being the essence of religion, would cause the religion to collapse and corruption and ignorance to pervade.
Hisbah derives relevance to freedom of expression since it entitles every Muslim to speak for a good cause or to disapprove and criticize an evil one. By virtue of hisbah, Islam makes it permissible for individuals to correct any religious or societal deviation. This entitlement cannot be exercised if individuals' freedom to speak or act is banned or limited. The imperative to act according to ḥisbah is found in the clear text of the Quran where God says, "And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful." The meaning intended from this verse is that there should be a group from among the Muslim community who take up this responsibility, making it, most preponderantly, a collective obligation.
Indicative of how closely it relates to individuals' rights and liberties, the principle of ḥisbah, both as a right and a duty, has found explicit recognition in the Universal Islamic Declaration of Human Rights. This document refers to ḥisbah as constituting 'the right and duty of every person' to speak for and defend the rights of others and those of the community when these are threatened or violated.
The exercise of ḥisbah is not left unregulated, for otherwise its exercise would be misused and would fail to meet its objectives. One guideline concerning enjoining good and forbidding evil is that a person must not enjoin good or forbid evil in case this would result in a greater evil or waste greater good. This regulation is in line with the legal maxim of "harm may not be inflicted nor reciprocated." What substantiates this prerequisite is that God Most High forbade the Prophet's companions from insulting the deities of the infidels although they were false gods, commanding: "And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge." This is because insulting the deities of the infidels would in turn make them insult God, which is an enormous offence. Al-Qurṭubi further notes that enjoining good or forbidding evil is incumbent only when a positive reaction is anticipated, and no violent reaction or harm is expected. Otherwise, an individual would only be responsible for himself and those dependent on him, based on the verse reading: "O you who have believed, upon you is [responsibility for] yourselves. Those who have gone astray will not harm you when you have been guided."
The exercise of ḥisbah must also be based on sound Shar‘i knowledge. By virtue of this stipulation, a person must be certain that the thing he is commanding to be observed is mandatory or preferable and that the thing he is forbidding is prohibited or abominable based on a Sharia evidence. In addition to knowledge, the person exercising ḥisbah must be considerate and must not use offensive or coarse language, for failing to observe this courtesy is likely to bring about adverse results. This courtesy is encouraged by the Prophet as he said, "Allah is Forbearer and He loves forbearance, and rewards for forbearance while He does not reward severity, and does not give for anything besides it (forbearance)."
To sum up, ḥisbah is a principle that constitutes a piece of the evidence in support of freedom of speech in Islam. However, ḥisbah is regulated within certain confines to ensure that its exercise neither fails to meet its intended objectives nor brings about adverse consequences.