Hadith Forgery (Arabic: جَعْل الحَدِيث) is the act of forging a hadith and attributing it to the Prophet (s) or an Infallible Imam (a), which has occurred in the form of fabricating an entire hadith, addition of some words or sentences to an existing hadith, or changing its wording. This phenomenon began at the time of the Prophet (s) and became a significant phenomenon during the reign of Mu'awiya.
Among the motivations and purposes of hadith forgery were overshadowing the virtues of Imam Ali (a), legitimizing the rule of Mu'awiya, sectarianism, depriving Ahl al-Bayt (a) of their position, and challenging authentic hadiths.
Many works have been written on hadith forgery, including al-Mawḍu'at (Forged Hadiths) by Ibn al-Jawzi (d. 597/1200-1) and Al-Akhbar al-dakhila (Fabricated Reports) by Shaykh Muhammad Taqi Shushtari (d. 1415/1995).
A forged hadith is a hadith that is composed and then attributed, intentionally or by mistake, to the Prophet (s) or an Infallible Imam (a). Among the signs of a forged hadith is its incompatibility with reason, the Quran, or an essential of the Religion.
Hadith forgery has had different types. In some cases, an entire hadith was fabricated and then attributed to the Prophet (s) or an Imam. Sometimes certain words or sentences were added to an existing hadith, and sometimes the words of an existing hadith were changed.
Fabricating a complete hadith has occurred with regard to doctrinal, ethical, historical, medical, hagiographical, and devotional matters. An instance of adding words to a hadith is the addition made by al-Mansur, the second Abbasid Caliph, to a hadith about the promised Mahdi. The original hadith read, "God will raise a man from my family, whose name is my name," and al-Mansur added to it the following words: "… and the name of his father is the name of my father" so that he could introduce his own son Muhammad as the promised Mahdi, because al-Mansur's given name, like that of the Prophet's father, was Abd Allah. 
An instance of forgery by way of changing the wording of a hadith is the following hadith: "When you saw Mu'awiya speaking on my pulpit, kill him," which was later change into "When you saw Mu'awiya speaking on my pulpit, receive him."
Plagiarism in hadith transmission, which is defined as putting oneself (or someone else) in place of an actual transmitter of a hadith in the chain of its transmitters is also counted as an instance of hadith forgery. 
The phenomenon of hadith forgery seems to have started during the Prophet's lifetime. The Prophet (s) reportedly warned that whoever attributes to him something he did not say will be doomed to hell. Instances of fabricated hadiths during the time of the Prophet (s) are found in historical and hadith sources.
Some Sunni scholars maintain that hadith forgery did not take place during the time of the first four caliphs but started after the martyrdom of Imam Ali (a), when various sects appeared, each trying tried to defend and promote itself in any possible way, including sometimes hadith forgery. 
It is said that hadith forgery became a significant phenomenon at the time of Mu'awiya. According to Ibn Abi l-Hadid, Mu'awiya would support hadith transmitters who forged hadiths praising Uthman or other Companions or criticizing Imam Ali (a). 
Countering Hadith Forgery
Toward the end of the fifth/eleventh century, a number of works were written on the issue of hadith forgery and fabricated hadiths, though accusations of forgery against certain hadith transmitters were made by earlier Shiite scholars such as Ibn Uqda and Ibn al-Walid al-Qummi in the beginning of the fourth/tenth century and Ibn Ghada'iri in the fifth/eleventh century. It is said that among Sunni hadith scholars, the accusation of forgery was first used against some hadith transmitters by the scholars of the school of Baghdad in their evaluation of hadiths. 
Motivations of Forgery
Hadith forgery was done with various motivations, some of which are the following:
- Overshadowing the virtues of Imam Ali (a): It is said that Imam Ali (a) was a figure against whom hadiths were forged more than anyone else. Ibn Abi l-Hadid reports that Mu'awiya charged a group of hadith transmitters with the task of forging hadiths against Ali (a). He asked his governors to call people to forge hadiths in praise of the first three caliphs and other companions such that there remains no virtues of Ali (a) except that there is a parallel virtue narrated for other Companions.
- Legitimizing the rulers: According to some scholars, the Umayyads and Abbasids used to forge hadiths in praise of their prominent figures or to claim that their rule was endorsed by the Prophet (s). For instance, in one of these forged hadiths, the following words are attributed to the Prophet (s): "The caliphate will be placed among the children of my uncle [Abbas]."
- Sectarianism: Adherents of various sects would sometimes use anything, including forged hadiths, to strengthen their respective sects.
- Defaming Islam. 
- Getting close to the rulers.
- Encouraging people to recite the Quran and perform other righteous deeds.
Some of the consequences of hadith forgery are the following:
- Difficulty of distinguishing authentic hadiths from fabricated ones.
- Casting doubt upon the authenticity of indubitable hadiths: Some scholars have used the existence of hadith forgery as a pretext to doubt the authenticity of some indubitable hadiths. For instance, the Sunni scholar Ibn Qayyim al-Jawziyya (d. 751/1350-1), who was a student of Ibn Taymiyya, considered Hadith al-Ghadir a forged hadith, whereas Hadith al-Ghadir is mutawatir (massively transmitted) and its authenticity is indubitable.
- Marginalizing Ahl al-Bayt (a): The fabricators of hadith would attribute some sayings to the Imams (a) that would repel people from them. Such hadiths were forged mostly by the Exaggerators.
- Development of the science of rijal: Hadith forgery led to the development of the science of rijal, which discusses the reliability of hadith transmitters and the authenticity of hadiths.
Forgers of Hadith
The fabricators of hadith have been introduced mostly in the works related to hadith sciences. Allama Amini has mentioned in his al-Ghadir the names of about seven-hundred hadith forgers together with some of their forgeries. Some of these fabricators are said to have forged more than one thousand hadiths. This section of al-Ghadir has been published independently by the title al-Wadda'un wa ahadithihim al-mawdu'a (Forgers and Their Forged Hadiths).
Among prominent hadith forgers are the following:
- Abu Hurayra: In hadith collections, more than 5374 hadiths can be found that are narrated by him, whereas he was with the Prophet (s) only for three years. This is why some of the Companions would object to him, such as Ali (a), Umar, and Uthman. A'isha also would criticize him saying, "He quotes some hadiths from the Prophet (s) that I never heard from him."
- Ka'b al-Ahbar: According to Sayyid Murtada Askari, most of the reports about the Jews and praises for the People of the Book and their holy places entered Muslim sources and hadith collections through him.
- Ubayy b. Ka'b: He narrated a hadith on the merits of reciting Quranic suras, but later he confessed that he had forged it.
- Nuh b. Abi Maryam al-Marwazi: He fabricated a hadith on the merits of reciting various suras and then explained that he did so because people had abandoned the Quran and turned to al-Fiqh by Abu Hanifa] and al-Maghazi by Ibn Ishaq. 
- Ibn Abi l-Awja': It is said that he forged about four-thousand hadiths.
Many works have been written on the issue of hadith forgery, most of which try to collect forged hadiths and list the forgers. Among these works are the following:
- Al-Mawdu'at (Forged Hadiths) by Ibn al-Jawzi (d. 597/1200-1)
- Al-La'ali l-masnu'a fi l-ahadith al-mawdu'a (Manufactured Pearls: on Fabricated Hadiths) by Jalal al-Din al-Suyuti (d. 911/1505-6)
- Al-Akhbar al-dakhila (Fabricated Reports) by Shaykh Muhammad Taqi Shushtari (d. 1415/1995)
- Al-Mawdu'at fi al-athar wa l-akhbar (Forged Hadiths among the Hadiths and Reports) by Sayyid Hashim Ma'ruf al-Hasani (d. 1404/1983-4)
- Khamsun wa mi’a sahabi mukhtalaq (One Hundred Fifty Fabricated Companions) by Sayyid Murtada Askari (d. 2007)
- Al-Wad' fi l-hadith (Forgery in Hadith) by Umar b Hasan Falata
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