Like all Muslims, Sufis have looked to the scripture of Islam for inspiration and justification for their particular interpretation of how to practice religion. A number of verses in the Quran are cited by Sufis to illustrate the legitimacy of an esoteric interpretation of scripture.
In Surah 24:35, the famous "Light Verse" reads: "Allah is the Light of Heaven and Earth. His light may be compared to a niche in which there is a lamp; the lamp is in a glass; the glass is just as if it were a glittering star kindled from a blessed olive tree (that is) neither Eastern nor Western, whose oil will almost glow though fire has never touched it. Light upon light, Allah guides anyone He wishes to His light."
Another verse that concisely encompasses the Sufi goal of returning to and uniting with God is 2:16: "Verily we are for Allah, and verily unto Him we are returning." To illustrate the close relationship between humans and God, another foundational verse is 50:16, in which God says of man: "We are nearer to him than his jugular vein."
Beyond specific verses such as those cited above, Sufis eventually penned their own Quranic commentaries, which were vast exegetical works expounding upon the meaning of each verse of the scripture. They based their interpretations on a hadith in which Muhammad said that every verse of the revelation had an outer and an inner meaning, or an exoteric and esoteric sense. Because the esoteric or inner meaning of verses was, by definition, not obvious, such readings of the Quran were open to the criticism that that Sufi commentators were reading more into the text than they were expounding on what was already there.
Perhaps in response to such accusations, Sufis also looked to the traditions about Muhammad in the form of hadith to validate their claims. Another hadith that served this purpose reads, "Even though I depart from the world, I leave you the Quran. That is your evidence, keep it close to you. The Quran will be a teacher and a sage to the innermost heart."
Although the literature and traditions upon which Sufis relied (aside from the Quran and hadith) were not always universally agreed upon by the mainstream community, the very fact that Sufi practitioners and thinkers resorted to tradition to legitimize their claims is telling. While Sufis occasionally faced some strong opposition, their rooting of their own belief system in the traditional literature and scripture of Islam has led to what may be seen as an uneasy relationship with those detractors. If Sufis based their practice on the Quran, they could hardly be attacked by other Muslims for doing so.
One example of a practice derived from a reading of the Quran is the dhikr ceremony or gathering. In practice, Sufis have elaborated various ways in which to gather for group observance and meditation in which repetitive supplications are made aloud. The basis for these dhikr sessions is verse 2:152, which reads, "Then do ye remember Me, I will remember you," in which the word "remember" has the same root as dhikr, which literally means "remembrance." Dhikr ceremonies currently take several forms, but the practice of communal gathering for incantation and supplication is a hallmark of Sufi practice across Muslim contexts.
By combining adherence to Quranic themes with a highlighting of several carefully chosen hadith, Sufism is firmly rooted in the traditional sources of mainstream Islam. Instead of composing or seeking an alternate scripture, Sufism contains alternate readings of the Quran and literature about Muhammad.