Hadith remembers God unceasingly. (Al-Ahzab 33: 21) The hadith literature is based on spoken reports in circulation after the death of Muhammad. Unlike the Qur'an, ahadith were not promptly written down during Muhammad's life or immediately after his death. Hadith were evaluated and gathered into large collections during the 8th and 9th centuries, generations after the death of Muhammad, after the end of the era of the "rightful" Rashidun Caliphate, over from where Muhammad lived. "Many thousands of times" more numerous than Quranic verse, ahadith have been described as resembling layers surrounding the “core” of the Islamic belief (the Quran). Well-known,
Introduction to the Science of Hadith al-Ṣalāḥ, in his "Introduction", codified the terminology established by those scholars before him based upon his reading of their works. He did so by citing some of those scholarly statements in the earlier works and deducing from them common terms and definitions in the manner of a scholar authoring a book of "fiqh" (jurisprudence). In this manner, his book was based upon those principles established by the earlier hadith specialists combined with some elements of "fiqh". An example of this would be the inclusion of the division of hadith into "mutawātir" and "āḥād". He therefore only mentioned the statements of