Wikipedia
Kitab al-Risāla Fī Uṣūl al-Fiqh, often abridged as Kitāb al-Risāla or simply al-Risāla, is the best known work of al-Shafi'i, noted especially for its clear Islamic jurisprudence. The word risāla in Arabic means a letter, or simply a communication in writing made to an absent person. Shafi'i's treatise received its name owing to a traditional, though unverified, story that Shafi'i composed the work in response to a request from a leading traditionist in Basra, ‘Abd al-Raḥmān bin Mahdī; the story goes that Ibn Mahdī wanted Shafi'i to explain the legal significance of the Quran and the sunna, and the Risāla was Shafi'i's response.
In this work, al-Shafi'i is said to have outlined four sources of Islamic law, though this division based on four has been attributed to later commentators on the work rather than to Shafi'i himself.