This article aims at examining the limits of change in the field of worship through a study on the origins of the hukm [religious ruling] of Shawwal fasting that is widely practiced in the different parts of Muslim world. The study, firstly, deals with the evolution of the hukm of Shawwal fasting chronologically among four sunni schools of law, then analyzes the solitary reports on the topic. It concludes that in Maliki and Hanefi schools, the hukm of this specific worship changed within the limits of karaha [reprehensibility], ibaha [permissibility], and nadb [recommendation]. This hukm was constructed based on solitary reports which later became a paradigmatic source of religious legitimacy in the Hanbali school beginning with the eponymous Imam and in the Shafi i long after the founding authorities. It also claims that it is strongly probable that the solitary reports on the topic go back to a deduced opinion in the age of companions, but then reached to a form of prophetic solitary report in the corpus of hadith reports. Ultimately, this specific worship that has restricted number of fasting was established with the paradigmatic effect of the approach to solitary reports as the only authentic way of transmitting the sunna, even though it is at odds with the fact that this worship had not been implemented in the first two centuries after hijra, i.e. contrary to the established practice [amal].