examines deficiencies in the text and/or the chain of a hadith; study of al-hadith al-muhtelif, which attempts to reconcile the contradictions of hadith; the study of naskh or nasikh and mansukh in hadith, which also attempts to reconcile contradictions in hadiths, but by determining which of the contradicting hadith abrogates the other; study of sharh al-hadith, which are commentary on hadith that attempt "to explain the intentions (of) Prophet Muhammad (in uttering it)"; study of ʿilm jarḥ wa taʿdīl (wounding and rectifying), which attempts to verify the reliability of transmitters of hadith, their deficiencies and virtues; study of transmitters of hadith, ʿilm al-rijāl (science of men) which provides biographies of the narrators and the different categories they fall under. History After the death of Muhammad, his sayings were transmitted orally. According to Islamic tradition, Umar ibn al-Khattab, the second caliph, started the process of collecting all the hadiths together into one unified volume, but gave up the endeavor "for fear the Quran would be neglected by the Muslims" (according to Muhammad Zubayr Siddiqi). The Umayyad caliph, Umar ibn Abd al-Aziz (aka Umar II, who reigned from 717-720 CE) also started an effort to collect all the hadiths. Teaching and collecting hadiths was part of a plan of his to renew the moral fiber of the Muslim community. He supported teachers of fiqh, sent educators to Bedouin tribes, ordered weekly hadith lectures in the Hejaz, and sent out scholars of hadith to Egypt and North Africa, (according to Muhammad Zubayr Siddiqi). Umar also ordered the great scholar of Madinah, Abu Bakr ibn Hazm to write down all the hadiths of Muhammad and Umar ibn al-Khattab, particularly those narrated by Aisha. He had these hadiths collected in books which were circulated around the Umayyad Empire. Although these books are lost today, commentaries on them by Ibn al-Nadim reveals that they are organized like books of fiqh, such as the Muwatta of Imam Malik, the first large compilation of hadiths. Imam Malik himself probably followed the general plan of the early books of hadith ordered by Umar. Hadith studies developed in part because forgery "took place on a massive scale", with perhaps the most famous collector of hadith and practitioner of ʻilm al-ḥadīth—Muhammad al-Bukhari—sifting through nearly 600,000, over 16 years before eliminating all but approximately 7400 hadith. Traditional accounts describe "the systematic study of hadith" as being motivated by the altruism of "pious