This paper focuses on the concept of sunna, and its great theoretical and historical complexity. The term “sunna” seems nowadays to be a label for the whole set of behaviors adopted by believers, from simple details of dress or diet to the most elaborate of ritual and cultural attitudes. The daily lifestyle of the prophet is invoked so as to imitate the way he washed, how he lay down to sleep, or “sat at table”, not to mention his ritual gestures during prayer, what he preferred to eat after breaking fasts, his slightest movements over the nights and days of pilgrimage. The author intends to show on the contrary, through an unprecedented study, that the Islamic society of a very large city, that of 10th century Baghdad, was for a considerable period of time completely disengaged from such obsessive scrupulousness of religious observance. It gives large evidence of this phenomenon, and tries to outline the historical reasons how the sunna eventually evolved into a rule defining the coercive power of the Islamic faith.