Though a prophetic saying (Hadith) claims that "he who knows God becomes silent," the Sufis have produced a literature of impressive extent and could defend their writing activities with another Hadith: "He who knows God talks much." The first systematic books explaining the tenets of Sufism date from the 10th century; but earlier, Muhasibi had already written about spiritual education, Hallaj had composed meditations in highly concentrated language, and many Sufis had used poetry for conveying their experiences of the ineffable mystery or had instructed their disciples in letters of cryptographic density. The accounts of Sufism by Sarraj and his followers, as well as the tabaqat (biographical works) by Sulami, Abu Nu'aym al-Isfahani, and others, together with some biographies of individual masters, are the sources for knowledge of early Sufism. (See Islamic literature.)
Early mystical commentaries on the Qur'an are only partly extant, often preserved in fragmentary quotations in later sources. With the formation of mystical orders, books about the behaviour of the Sufi in various situations became important, although this topic had already been touched on in such classical works as Adab al-muridin ("The Adepts' Etiquette") by Abu Najib as-Suhrawardi (died 1168), the founder of the Suhrawardiyah order and uncle of the author of the oft translated 'Awarif al-ma'arif ("The Well-known Sorts of Knowledge"). The theosophists had to condense their systems in readable form; Ibn al-'Arabi's al-Futuhat al-Makkiyah ("The Meccan Revelations") is the textbook of wahdat al-wujud (God and creation as two aspects of one reality); his smaller work on the peculiar character of the prophets--Fusus al-hikam ("The Bezels--or cutting edges--of Wisdom")--became even more popular.
Later mystics commented extensively upon the classical sources and, sometimes, translated them into their mother tongues. A literary type that has flourished especially in India since the 13th century is the malfuzat, a collection of sayings of the mystical leader, which are psychologically interesting and allow glimpses into the political and social situation of the Muslim community. Collections of letters of the shaykhs are similarly revealing. Sufi literature abounds in hagiography, either biographies of all known saints from the Prophet to the day of the author, or of saints of a specific order, or of those who lived in a certain town or province, so that much information on the development of Sufi thought and practice is available if sources are critically sifted.
The greatest contribution of Sufism to Islamic literature, however, is poetry--beginning with charming, short Arabic love poems (sometimes sung for a mystical concert, sama') that express the yearning of the soul for union with the beloved. The love-relation prevailing in most Persian poetry is that between a man and a beautiful youth; less often, as in the writings of Ibn al-'Arabi and Ibn al-Fari, eternal beauty is symbolized through female beauty; in Indo-Muslim popular mystical songs the soul is the loving wife, God the longed-for husband. Long mystic-didactic poems (masnavis) were written to introduce the reader to the problems of unity and love by means of allegories and parables. After Sana'i's (died 1131?) Hadiqat al-haqiqah wa shari'at at-tariqah ("The Garden of Truth and the Law of Practice"),came 'Attar's Manteq ot-teyr ("The Birds' Conversation") and Rumi's Masnavi-ye ma'navi ("Spiritual Couplets"). These three works are the sources that have furnished poets for centuries with mystical ideas and images. Typical of Sufi poetry is the hymn in praise of God, expressed in chains of repetitions.
The mystics also contributed largely to the development of national and regional literatures, for they had to convey their message to the masses in their own languages: in Turkey as well as in the Panjabi-, the Sindhi-, and the Urdu-speaking areas of South Asia, the first true religious poetry was written by Sufis, who blended classical Islamic motifs with inherited popular legends and used popular rather than Persian metres. Sufi poetry expressing divine love and mystical union through the metaphors of profane love and union often resembled ordinary worldly love poetry; and nonmystical poetry made use of the Sufi vocabulary, thus producing an ambiguity that is felt to be one of the most attractive and characteristic features of Persian, Turkish, and Urdu literatures. Sufi ideas thus permeated the hearts of all those who hearkened to poetry. An example is al-Husayn ibn Mansur al-Hallaj, the 10th-century martyr-mystic, who is as popular in modern progressive Urdu poetry as he was with the "God-intoxicated" Sufis; he has been converted into a symbol of suffering for one's ideals.