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-Kattānī. `Āʼishah bint `Abd al-Raḥmān said in the foreword of her edition of the "Introduction": His book about the sciences of hadith is the best known of his works without comparison, to the extent that it is sufficient to say, "the Book of Ibn al-Ṣalāḥ", the intent being understood due to its popularity and stature. With the previous scholars, its subject matter overcame it, thus being referred to as, "the Book of Ibn al-Ṣalāḥ about `Ulūm al-Ḥadīth" or as he referred to it in its opening pages, "the Book of Familiarity with the Types of `Ulūm al-Ḥadīth". It has become