According to hadith حَدِيث literature, the Quran is revealed in seven Ahruf أَحْرُف (the plural of harf حَرْف). Ahruf are distinct from Qira'at قِرَاءَات. Malik Ibn Anas reported that the second Rashidun Caliph Umar Ibn al-Khattab said: "I heard Hisham Ibn Hakim Ibn Hizam reading Surat Al-Furqan in a different way from the one I used to read it, and the Prophet himself had read out this surah to me. Consequently, as soon as I heard him, I wanted to get hold of him. However, I gave him respite until he had finished the prayer. Then I got hold of his cloak and dragged him to the Prophet. I said to him: “I have heard this person [Hisham Ibn Hakim Ibn Hizam] reading Surah Al Furqan in a different way from the one you had read it out to me.” The Prophet said: “Leave him alone [O ‘Umar].” Then he said to Hisham: “Read [it].” [Umar said:] “He read it out in the same way as he had done before me.” [At this,] the Prophet said: “It was revealed thus.” Then the Prophet asked me to read it out. So I read it out. [At this], he said: “It was revealed thus; this Quran has been revealed in seven Ahruf. You can read it in any of them you find easy from among them."
Comparison to Qira'at
Bilal Philips writes that the Quran continued to be read according to the seven ahruf until midway through Caliph 'Uthman's rule when some confusion arose in the outlying provinces concerning the Quran's recitation. Some Arab tribes had begun to boast about the superiority of their ahruf and a rivalry began to develop. At the same time, some new Muslims also began mixing the various forms of recitation out of ignorance. Caliph 'Uthman decided to make official copies of the Quran according to the writing conventions of the Quraysh and send them along with the Quranic reciters to the major centres of Islam. This decision was approved by Sahaabah and all unofficial copies of the Quran were destroyed. Uthman burned the unofficial copies of the Quran. Following the distribution of the official copies, all the other ahruf were dropped and the Quran began to be read in only one harf. Thus, the Quran which is available throughout the world today is written and recited only according to the harf of Quraysh.
On Qirâ'ât, Philips writes that it is for the most part a method of pronunciation used in the recitations of the Quran. These methods are different from the seven forms or modes (ahruf) in which the Quran was revealed. The seven modes were reduced to one, that of the Quraysh, during the era of Caliph 'Uthman, and all of the methods of recitation are based on this mode. The various methods have all been traced back to the Prophet through a number of Sahaabah who were most noted for their Quranic recitations. That is, these Sahaabah recited the Quran to the Prophet or in his presence and received his approval. Among them were the following: Ubayy Ibn K'ab, Ali Ibn Abi Taalib, Zayd Ibn Thaabit, 'Abdullah Ibn Mas'ud, Abu ad-Dardaa and Abu Musaa al-Ash'aree. Many of the other Sahaabah learned from these masters. For example, Ibn 'Abbaas, the master commentator of the Quran among the Sahaabah, learned from both Ubayy and Zayd. (pp. 29–30)
On transmission of Quran, Philips writes that among the next generation of Muslims referred to as Tabi'in, there arose many scholars who learned the various methods of recitation from the Sahaabah and taught them to others. Centres of Quranic recitation developed in al-Madeenah, Makkah, Kufa, Basrah and Syria, leading to the evolution of Quranic recitation into an independent science. By mid-eighth century CE, there existed a large number of outstanding scholars all of whom were considered specialists in the field of recitation. Most of their methods of recitations were authenticated by chains of reliable narrators ending with the Prophet. Those methods which were supported by a large number of reliable narrators on each level of their chain were called Mutawaatir and were considered to be the most accurate. Those methods in which the number of narrators were few or only one on any level of the chain were referred to as shaadhdh. Some of the scholars of the following period began the practice of designating a set number of individual scholars from the previous period as being the most noteworthy and accurate. By the middle of the tenth century, the number seven became popular since it coincided with the number of dialects in which the Quran was revealed. (pp. 30)
Javed Ahmad Ghamidi, on the other hand, while commenting on hadith in Muwatta, writes that if Ahruf are taken in the context of pronunciation (for which actual words are lughat and lahjat), then the content of the hadith rejects this meaning itself as it is known that Umar and Hisham belonged to the same tribe - Quraysh, and people from same tribe cannot have different pronunciation. Hence, he questions those hadith which purport "variant readings". He also insists on the basis of Quranic verses ([Quran 87:6-7], [Quran 75:16-19]) that Quran was compiled in the life of Muhammad, hence he questions those hadith which report compilation of Quran in Uthman's period: As most of these narrations are reported by Ibn Shihab al-Zuhri, Imam Layth Ibn Sa‘d in his letter to Imam Malik has written:
And when we would meet Ibn Shihab, there would arise a difference of opinion in many issues. When any one of us would ask him in writing about some issue, he, in spite of being so learned, would give three very different answers, and he would not even be aware of what he had already said. It is because of this that I have left him – something which you did not like.
It is said that Abu ‘Ubayd Qasim Ibn Sallam (d. 224 AH) selected twenty five readings in his book. The seven readings which are famous in current times were selected by Abu Bakr Ibn Mujahid (d. 324 AH) at the end of the third century hijrah. Thus it is generally accepted that their number cannot be ascertained but every reading is Quran which has been reported through a correct chain of narration, are found in any way in the masahif prepared by ‘Uthman and are correct from any aspect as far as the Arabic language is concerned. Some of these readings are regarded as mutawatir; however, a look at their chains of narration which are found in books leaves no doubt that they are ahad (isolate), most narrators of which are suspect in the eyes of the rijal authorities.
Testimony by Medinans and 'Uthman
'Uthman Ibn ‘Affan asked those present at the mosque of Madinah if any of them had ever heard Muhammad say, "The Qur'an has been revealed to be recited in seven ahruf." In response, a huge number of them stood up and testified that they had heard this hadith. Consequently, 'Uthman himself emphasized this hadith by stating that he testified with them. 
- Malik Ibn Anas, Muwatta, vol. 1 (Egypt: Dar Ahya al-Turath, n.d.), 201, (no. 473).
- Abu Ameenah Bilal Philips, Tafseer Soorah Al-Hujuraat, 1990, Tawheed Publications, Riyadh, p. 28-29
- Javed Ahmad Ghamidi. Mizan, Principles of Understanding the Qu'ran Archived September 27, 2007, at the Wayback Machine, Al-Mawrid
- Ibn Qayyim, I‘lam al-Muwaqqi‘in, vol. 3 (Beirut: Dar al-Fikr, n.d.), 96.
- Nashr 1:21 and al-Musnad al-Kabir 1:131