Under the Sharia, threats, regardless of whether they are made seriously, in jest or in gesture receive emphatic prohibition. The term "threatening" (Ar. tahdi̅d) can rarely be found in the classical juristic manuals, yet it may be subsumed under the topic of tarwi̅‘ (intimidation) since threats and intimidation are concurrent, for threats usually cast fear in the hearts of those against whom it is made.
In the literal sense, tarwi̅‘ is derived from the base-form verb rawwa‘a meaning to terrify or intimidate someone. The prohibition of threats is evidenced by the hadith where the Prophet said, "None amongst you should point a weapon towards his brother, for he does not know that Satan might cause the weapon (to slip) from his hand and (he may injure anyone) and thus he may fall into Hell-Fire." [Al-Bukhari] Accordingly, Ibn Hajar Al-Haitami classified horrifying a Muslim as a major sin in a separate section of one of his works.
This prohibition extends to unintentional threats, for it is reported that a person happened to come to the mosque with an arrow; thereupon, the Prophet said to him: 'Take hold of its pointed head.' [Al-Bukhari] Although it was apparent that the man intended no harm, the Prophet instructed him man to cover the pointy ends of his arrows out of concern that this would threaten and intimidate the people who were present. In line with the above hadith, the Prophet declared: "He who points a weapon towards his brother the angels invoke curse upon him even if he is his real brother so long as he does not abandon it (the pointing of weapon towards one's brother Muslim)." [Muslim] Commenting on this hadith, An-Nawawi states that it emphasizes the invulnerability of a Muslim and the strict prohibition of horrifying him, and that the phrase "even if he is his real brother" indicates the generic prohibition of this behavior and that it applies in absolute terms. He further maintains that this behavior is prohibited even if exercised in jest.
Not only that, but threats made by gesture are also prohibited. It is reported that the Prophet said: "He who looks at a Muslim in such a way to intimidate him unrightfully shall be intimidated by Allah on the Day of Judgment." [Al-Tabarani] This shows the extent to which Islam forbids this variety of expression, for, besides the prohibition mentioned in this hadith and the aforementioned ones, it also merits a punishment after Resurrection.
Islam took an emphatic position on cases where a person terrifies the others. Al-Mana̅wi states that terrifying a Muslim is strictly prohibited, and constitutes a major sin (kabi̅rah). Al-Manawi's classification is based on the Prophet's hadith where he declared: 'It is not lawful for a Muslim that he frightens a Muslim.' [Abu̅ Dawu̅d]
The Sharia does not enforce a fixed penalty for threats. Yet, it may be penalized under the penal authority of ta‘zi̅r. However, it is argued that threats which are tantamount to terrorism may be penalized with the fixed punishment proscribed for the crime of ḥira̅bah. Ḥira̅bah is generally defined as the crime setting out to rob, murder or terrify people in public places. The penalty of ḥira̅bah is evidenced by the verse reading: "The punishment for those who wage war against God and His Prophet, and perpetrate disorders in the land, is to kill or hang them, or have a hand on one side and a foot on the other cut off, or banish them from the land." [Quran 5:33] The Shafi‘is and Ḥanbalis are of the view that he who only terrorizes people but does not kill or rob is to be punished by banishment, with Ḥanafis maintaining that nafi (banishment) in this verse means imprisonment since it is impossible to banish someone from the whole world.
Based on the above evidence, it is clear that threats is a prohibited form of expression under the sharia and this prohibition further conforms to the Islamic principle of blocking the means to expected evil (Sadd Adh-Dhara̅e‘), for a threat, even when made in jest, may materialize, leading to a greater evil and grave consequences.