Imam Abu Dawud He compiled twenty-one books related to Hadith and preferred those ahadith which were supported by the example of the companions of Muhammad. As for the contradictory ahadith, he states under the heading of 'Meat acquired by hunting for a pilgrim': "if there are two contradictory reports from the Prophet (SAW), an investigation should be made to establish what his companions have adopted". He wrote in his letter to the people of Makkah: "I have disclosed wherever there was too much weakness in regard to any tradition in my collection. But if I happen to leave a Hadith without any comment, it should be considered as sound, albeit some of them are more authentic than others". Hadith Mursal (a tradition in which a companion is omitted and a successor narrates directly from Muhammad) has also been a matter of discussion among the traditionists. Imam Abu Dawud states in his letter to the people of Makkah: "if a Musnad Hadith (uninterrupted tradition) is not contrary to a Mursal or a Musnad Hadith is not found, then the Mursal Hadith will be accepted though it would not be considered as strong as a Muttasil Hadith (uninterrupted chain)".
The traditions in Sunan Abu Dawud are divided in three categories. The first category consists of those traditions that are mentioned by Imam Bukhari and/or Imam Muslim. The second type of traditions are those which fulfil the conditions of Imam Bukhari or Imam Muslim. At this juncture, it should be remembered that Imam Bukhari said, "I only included in my book Sahih Bukhari authentic traditions, and left out many more authentic ones than these to avoid unnecessary length". He had no intention of collecting all the authentic traditions. He only wanted to compile a manual of Hadith. according to the wishes of his teacher Ishaq b. Rahaway (d. 238A.H.), and his function is quite clear from the complete title of his book Al-Jami', al-Musnad, al-Sahih, al-Mukhtasar, Min 'Umur Rasul Allah Wa-Sunanihi Wa-Ayyamihi. The word al-Mukhtasar (epitome), itself explains that Imam Bukhari did not attempt to compile a comprehensive collection. Imam Muslim said that he condensed his Sahih from 300,000 Sahih ahadith. This proves that there are many ahadith which are authentic in spite of their not being mentioned in either Sahih Bukhari or Sahih Muslim.
Abu Dawud collected 500,000 hadith, but included only 4,800 in this collection. Sunnis regard this collection as fourth in strength of their Six major Hadith collections. It took him 20 years for collecting the ahadis. He made series of journeys to meet most of the foremost traditionists of his time and acquired from them the most reliable ahadis quoting sources through which it reached him. Since the author collected ahadis which no one ever assembled together, his sunan has been accepted as standard work by scholars from many parts of the Islamic world, especially after Ibn al-Qaisarani's inclusion of it in the formal canonization of the six major collections.