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 authored an abridgement: The following have set "Introduction" to verse, adding some content in the process: The numerous editions of the "Introduction" in its original Arabic include two of the more reliable: The "Introduction" has been translated into English and published as "An Introduction to the Science of Hadith", by Eerik Dickinson, as part of the Great Books of Islamic Civilization series. The translator has provided a biography of Ibn al-Ṣalāḥ derived from numerous sources, in addition to copious footnotes throughout. It is published by Garnet Publishing Limited, Reading, 2006, 356 pgs. Introduction to the Science of Hadith (Ibn al-Ṣalāḥ's)