In works on Islam the word "hadith" usually refers to the sayings or "traditions" which have been transmitted from the Prophet. Muslims hold these to be the most important source of Islamic teachings after the Qur'an. Numerous works have been written in Western languages on the role of the hadith literature in Islam  and a number of important translations have been made.  But almost all Western studies have been limited to the point of view of Sunni Islam and based on Sunni sources and collections. Practically no one has paid any serious attention to the different nature of the hadith literature in Shi'ism and the different sources from which the hadiths are derived.
The fundamental distinction to be made between Shi'ite and Sunni hadiths is that in Shi'ism the traditions are not limited to those of the Prophet, but include those of the Imams as well. As important and basic as this point is, it has not been understood even in such standard reference works as the new Encyclopedia of Islam. There the author of the article "Hadith" is aware that there is some difference between Shi'ism and Sunnism on the question of which hadiths are included, but he thinks that it lies in the fact that the Shi'ite collections accept "only traditions traced through 'Ali's family." But this is incorrect, since numerous traditions are also transmitted through other sources. What the author fails to mention is that the hadith literature as understood by Shi'ites is not limited to the sayings of the Prophet, but includes those of the Imams as well. 
In short, collections of hadiths in Sunni Islam, such as those of al-Bukhari and Muslim, contain only sayings transmitted from and about the Prophet. But the Shi'ite collections, such as that of al-Kulayni, also contain sayings transmitted from and about the twelve Imams. Naturally the Shi'ites make a distinction among the hadiths, so that those transmitted from the Prophet are of greater authority, but nevertheless all traditions are listed together according to subject matter, not according to author.
The most famous and authoritative collections of Shi'ite hadiths are four works which, in terms of their importance for Shi'ism, correspond to the Six Correct Collections in Sunni Islam. These are al-Kafi fi 'ilm al-din (The Sufficient in the Knowledge of Religion) by Thiqat al-Islam Muhammad ibn Ya'qub al-Kulayni (d. 329/940), Man la yahduruhu al-faqih (For him not in the Presence of Jurisprudent) of Shaykh al-Saduq Muhammad ibn Babuyah al-Qummi (d. 381/991), Tahdhib al-ahkam (Rectification of the Statutes) by Shaykh al-Ta'ifah Muhammad al-Tusi (d. 460/ 1068) and al-Istibsar fi ma ukhtulif fihi min al-akhbar (Reflection upon the Disputed Traditions) also by al-Tusi.
The Present Collection
The sermons, sayings, prayers and writings translated here present a cross section of Shi'ite religious thought with an emphasis upon that which is most basic for the religion itself and most universal and hence understandable in the eyes of non-Muslims. As 'Allamah Tabataba'i points out in his foreword, in making these selections his aim was to emphasize the three basic dimensions of the Shi'ite tradition: I. The profession of Unity (altawhid), or the metaphysical and theological principles of the faith 2. The political, social and moral teachings. 3. The inward, spiritual and devotional life of the community. Hence the selections stress the principles and fundamentals (usul) of Islam, while they tend to ignore the branches and secondary aspects (furu). In other words, little is said about the concrete ramifications of the principles in terms of the details of the application of the Divine Law (al-Shari'ah) to everyday life. Nevertheless, the secondary aspects are clearly reflected in 'Ali's "Instructions to Malik al-Ashtar" and to a lesser degree in the prayers.
Although it is well known that the first "pillar of Islam" is the profession of faith, which begins with a statement of the Divine Unity, Western scholars have tended to explain the Islamic belief in God's Oneness as a relativity simple-minded affirmation of the existence of only one God. Perhaps one reason the Nahj albalaghah and the Shi'ite hadith literature in general have been neglected or simply branded as spurious is that their very existence flatly contradicts the commonly accepted idea of a simple bedouin faith with few philosophical or metaphysical overtones. In these writings we see that already in the first centuries of Islam the Divine Unity was affirmed in terms reminiscent of the subtlety of later "theosophical" Sufism, but still completely steeped in the peculiar spiritual aroma of the revelation itself.
In making the selections 'Allamah Tabataba'i utilized four works: the Nahj al-balaghah, al-Sahifat al-sajjadiyyah, Bihar al-anwar and Mafatih al-jinan. The first two works are discussed in Dr. Nasr's introduction. Bihar al-anwar (Oceans of Lights) is a monumental encyclopedia of hadiths which attempts to collect all Shi'ite traditions in a single work and which classifies them by subject matter. It was compiled in the Safavid period by the famous theologian Muhammad Baqir Majlisi (d. 1110/1698-9 or 1111/1699-1700). The importance the work has possessed since its compilation as the standard reference work for all Shi'ite studies can hardly be overemphasized. One indication of its popularity is that, despite its enormous size, it was published twice in lithographed form in the nineteenth century. The modern edition of the work fills 110 volumes of approximately 400 pages each.
Majlisi collected his traditions from numerous earlier sources. As examples, we can mention a few of the works from which he derived the hadiths in the present collection, works which have been independently published in modern times. Shaykh al-Saduq, the author of one of the four basic works on Shi'ite hadiths referred to above, compiled dozens of authoritative hadith collections, each of which usually follows a particular theme. His al-Tawhid collects traditions which illustrate the profession of God's Unit. His 'Uyun akhbar al-Rida gathers together everything that has been related about Imam 'Ali al-Rida, the eighth Imam, whose tomb in Mashhad is the holiest site of pilgrimage in Iran. The work contains such things as descriptions of the Imam's mother, explanations of the reason his name was chosen, all the sayings which have been recorded from him, and traditions concerning his death and the miracles which have occurred at his tomb. Shaykh al-Saduq's al-Khisal demonstrates the importance of numbers in the traditions. In twelve long chapters he records all the hadiths which mention the numbers one to twelve. The author of al-Ihtijaj, Abu Mansur Ahmad ibn 'al-Tabarsi (d. 599/1202-3), rejects the views of certain of his contemporaries who had claimed that the Prophet and the Imams never engaged in argumentation. He collects together traditions in which their discussions with opponents have been recorded.
The fourth work from which 'Allamah Tabataba'i made his selections is Mafatih al-jinan ("Keys to the Gardens of Paradise"), a standard collection of Shi'ite prayers compiled from Bihar al-anwar and other sources by 'Abbas Qummi (d. 1359/1940-1). It includes prayers to be recited daily, prayers for special occasions such as religious holidays and days of mourning, litanies and invocations for different moments in one's life, instructions for making a pilgrimage to the tomb of the Prophet or any one of the Imams and prayers for every other conceivable occasion as well.
A note needs to be added about the method of translation. Because of the sacred nature of the texts and their fundamental importance as sources for the Shi'ite branch of Islam, I have attempted to translate them in a strictly literal manner so that the least amount of personal interpretation will have been made. There are definite disadvantages to this method, but the necessity for an accurate translation would seem to outweigh them all. After all, the Quran has been translated dozens of times. Others who may feel that the present translation does not do justice to the literary qualities of the text may try their own hand at rendering it into English.
The necessity for a literal translation is all the greater because a good deal of the material translated here-in particular those parts which derive from the Nahj al-balaghah-has also been translated elsewhere and on the whole has been misrepresented. Before such interpretive translations are made and held to reflect the thought of the Imams, literal translations are of paramount importance. In order to maintain a faithful translation, I have added notes wherever I deviate from a strictly literal translation or wherever there are questionable readings in the original.
Because no standard translations exist for many technical terms, I have felt it necessary to add the Arabic original in brackets for the benefit of scholars and Arabic speakers. This is especially true in the most difficult and metaphysical section of the book, Part I "On the Unity of God." Although the Arabic terms will prove a distraction to most readers, they represent the only practical way of tying the present texts into the reader's knowledge of the Arabic language.
Finally I would like to express my sincere gratitude to Seyyed Hossein Nasr, who asked me to undertake this work many years ago and has guided me in every stage of it, although of course I remain completely responsible for any inaccuracies which may remain in the translations and notes. Peter Lamborn Wilson and William Shpall also read the manuscript and made valuable suggestions. And without the kindness and encouragement of Wg. Cdr. (rtd.) Husayn and the Muhammadi Trust, the work may never have been completed and published.
 On the subject of Prophetic Hadith in general see the article "Hadith" in the Encyclopedia of Islam (new edition), where a good bibliography is also provided (vol. III, pp.23-8)
 Perhaps the most important hadith collection yet to be completed into worthy English is the Mishkat al-masabih, trans. By J. Robson, Lahore 4 vols, 1963-5. See also the translation of Bukhari mentioned in note 1 of the introduction, and Sahih Muslim, trans. By A.K Siddiqi, Lahore, 1972 onward.
 The Encyclopedia of Islam, vol. III, p.24.