Investigating the Shafi'i Reasons Regarding Authenticity of Sunna

Of the significant issues in the field of resons for inferencing the rules among Muslim scholars is the authenticity of Holy Prophet's Sunna and accepting or rejecting the Akhbar Aahaad (Hadiths transmited by just one person) abouth which the adherents and opponents have posed reasons during history. Shafi'i is one of the adherents of authenticity of Sunna and Akhbar Aahaad among the scholars of his age who, suggesting arguments in his writings, has defended the stance of Sunna. Mentioning arguments from religious texts and rational reasons, he states the authenticity of Sunna in general and that of Khabar Wahid  in particular and says inaddition to indication of Quranic verses to authenticity of Sunna and succinct expression of Quran, there are rules in Sunnah, besides what is mentioned in Quran, whaich are called Sunnah Za'id (extra Sunna). Apparently, this attitude of Shafi'i is not in tandem with that of some other scholars including Shatebi.  This article tends to delve into the arguments of the oponents of the authenticity of Sunna and Khabar Wahid  during Shafi'i era and present time and then states Shafi'i's attitude considering his arguments and poiting to the Quranic verses and authentic Hadiths regarding the authenticity of Sunna and poiting to religious and historical texts and considering the behavior of Prophet's companions, investigates the authenticity of Khabar Wahid  based on Shafi'i's terms.