Both Sunni and Shiite traditions record a statement of Muhammad according to which he left behind two weighty things. While both agree that the first one is the Quran, the Sunnis understand the second thing to be his exemplary traditions (sunna), whereas according to the Shiites, this second source of religious knowledge are his descendants.
This difference encapsulates much of the basic distinction between Sunnis and Shiites. It also characterizes their different approaches to the Quran, which strictly speaking, is the only scripture in both traditions. More precisely, Shiite approaches to the Quran differ from those of the Sunnis in two respects: 1) the exact wording of the text, where the significance of the descendants of Muhammad lies mainly in their historical role, and 2) the predominant exegetical tradition, where they almost assume the function of a hermeneutical tool.
The exact date and circumstances of the compilation of the Quran as it exists today are controversial. While some modern scholars posit an earlier or much later date than the Islamic tradition that credits the third caliph, Uthman, with the compilation of the authoritative version of the text, many others consider this a plausible scenario. It is much less controversial that during the first century or so of Islamic history variants of the Quran circulated, some of which were recorded as such and are still visible in modern printed texts, whereas others were suppressed. Among these was, according to the early Shiite tradition, the original version of the Quran as preserved by Ali, which included explicit passages in favor of him and the descendants of the Prophet.
The companions of Muhammad rejected this version and tampered with the text as—according to the Islamic tradition—Jews and Christians had done with previous revelations. This practice, known as tahrif, included the omission or manipulation of said passages. According to the current wording, for example, verse 33 of the third sura of the Quran states that "God has chosen Adam, Noah, the family of Abraham and the family of Imran above all beings," but Shiite tradition until the 9th-10th centuries held that the family of Muhammad had originally also been mentioned. Likewise, where verse 110 of the same sura reads in the standard version "You are the best of people," the actual text, according to earlier Shiite claims, had "You are the best of Imams." For modern readers, Shiite commentaries on the Quran are not always clear regarding the extent of manipulation they assume had taken place. While the references to Ali that they include in the text may have been believed to be part of the original version, exegetes may also have simply wanted to explain more general or obscure passages.
Toward the end of the Shiite intermezzo, in the 11th and 12th centuries, new compromises marked the relationship between Sunnis and Shiites, which included the general Sunni acceptance of the four caliphs as the "rightly guided" ones. On the Shiite side, the initial accusations of tahrif were largely abandoned, although they were and still are occasionally revived. Beyond the story about Ali's original version of the Quran, there is virtually no evidence that Shiites ever used what they thought to be this version. Shiites seem to have accepted the Uthmanic codex just as they, for the most part, accepted what they perceived to be the unjust rule of the Sunnis. And just as they had to wait for the Mahdi to restore justice, they had to wait for him to restore the original version of the Quran.
The exegetical tradition of Shiism is often characterized by an esoteric approach (tawil), a trend it shares with Sufi interpretations of the Quran. The text in itself offers only the outer meaning and principles. It is the task of the Imams as the 'speaking book of God' to reveal the inner meaning and the details. In Shiite commentaries on the Quran, the hidden meaning of the text is seen in its references to Shiite salvation history and the Imams. Negative statements, even to the devil, are sometimes interpreted as references to historical figures such as the first three caliphs, Aisha, or the Umayyads. Furthermore, different layers of hidden meanings are distinguished that correspond to different levels in the hierarchy of humans. The enigmatic nature of the text is often reflected in exegesis. The more hidden the truth is in the Quran, the more it needs to be disguised in the commentary. Both Sufi and Shiite exegeses have been attracted by the imagery of light in the Quran. On the one hand it could be understood as a manifestation of the divine, on the other hand its illuminating capacity could be seen as guidance for the believers. The blessed olive tree in the Light Verse (Quran 24:35), for example, is interpreted as a symbol for the Imams.