The extant Arabic sources include lots of hadith accounts concerning the proper performance of religious rituals. These accounts are sourced back to Muhammad through chains of transmission, and their reliability has been the most significant issue in the evolution of Islamic religious doctrine in the 20th century. In recent years, however, scholars working mainly in Europe have determined that if forgery was taking place on a large scale, it involved not only the names of authorities, but convincingly developing complete intellectual biographies that were shared among the occasionally inconvenient for the scattered generation of account transmitters who allegedly did the forging.
Based on this evidence, Haider uses the substantial corpus of traditions discussing religious rituals to develop three case studies to determine the degree to which Imamis, Zaydis, and Sunnis followed distinct paths in the crucial early 700's. He finds that they did, though the emergence of Zaydism over the period followed a somewhat different trajectory than has customarily been held.
In the final section of Haider's book, he examines the ways in which these identities were articulated in society. The materials concerning rituals and authorities on ritual matters suggest that how one performed acts of worship was, to some set of the public, more important than their specialized theological doctrines. These differences ultimately led to the development of separate sacred spaces within Kufa, the Iraqi city which was critical to early Shi'ism and the focus of Haider's study.
This whole book is not only important for its methodology and conclusions about early Shi'ism, but sits comfortably alongside other works examining religious identity in the late antique and early Islamic Middle East. I'm thinking here of Leor Halevi's Muhammad's Grave: Death Rites and the Making of Islamic Society which won MESA's most prestigious book award for its examination of how rituals surrounding death became and important symbol of the emerging Islamic identity and values from the 7th through 9th centuries, as well as Maged Mikhail's 2004 dissertation "Egypt from Late Antiquity to Early Islam: Copts, Melkites, and Muslims shaping a New Society," which received "Honorable Mention" for MESA's dissertation award. Although I can't find it now, I'm sure the latter work argued at some point that Egypt's Christian communities came to consolidate their separate identities largely through the observation of specific fasts and similar observances. One might also lump in here that in the 4th century John Chrysostom's Adversus Judaeos sermons were motivated by his desire to keep Christians from following Jewish observances. The common link among these three works is the consolidation of religious identity among communities of practice, and lens which may finally allow historians to better understand the social dimensions of the theological tumult of the period.