Sunan Abī Dāwūd is a collection of ḥadīth compiled by Imām Abū Dāwūd Sulaymān ibn al-Ash`ath as-Sijistānī (raḥimahullāh). It is widely considered to be among the six canonical collections of ḥadīth (Kutub as-Sittah) of the Sunnah of the Prophet ﷺ. It consists of 5239 aḥādīth in 43 books.
Author
Abū Dāwūd Sulaymān ibn al-Ash`ath ibn Isḥāq ibn Bashīr ibn Shaddād ibn `Amr ibn `Imrān al-Azdī as-Sijistānī was born in the year 202 AH in Sijistan, a province in Khurasan (a region covering parts of present-day Iran and Afghanistan). He was from the Azd tribe of Yemen.
He began to travel seeking aḥādīth at a young age, and reached Baghdad in 220 AH when he was 18 years old. He traveled for the purpose of checking out the possessors of aḥādīth to ensure their reliability before accepting their narrations. His journeys in search of knowledge took him through the lands of Khurasan, Iraq, Hijaz, Sham, Egypt, and Nishapur. He was not only a muḥaddith, but also a prominent faqīh who studied extensively under Imām Aḥmad ibn Hanbal.
His Virtues
Imām an-Nawawī states: “All the scholars are unanimous on the rank and virtue of Abū Dāwūd. He has been praised for his perfect memory and deep knowledge. He possessed a sound mind and perfect intellect. Moreover, he was an Imām in the various sciences. All of this, along with his piety and abstinence made him one of the greatest scholars of his time.” Adh-Dhahabī said: “Abū Dāwūd was among the greatest of scholars. It has been said that Abū Dāwūd resembled Aḥmad ibn Ḥanbal in manner, conduct and habits. And in that, Aḥmad resembled Wakī`, who resembled Sufyān, who resembled Manṣūr, who resembled Ibrāhīm an-Nakha`ī, who resembled `Alqamah, who resembled `Abdullāh ibn Mas`ūd. `Alqamah said: ‘`Abdullāh ibn Mas`ūd resembled the Prophet ﷺ in his manners, conduct and habits.’”
Once Imām Abū Dāwūd was on board a ship when he heard a person on the shore sneeze and say "Alhamdulillah" (all praise and gratitude belongs to Allāh alone). He hired a smaller boat for one dirham to row all the way to the shore just to reply "Yarhamukallah" (may Allāh have mercy on you) to the person’s sneeze. On returning, when asked about his action, he answered: “It is possible that the person who sneezed was one whose supplications are readily accepted by Allāh.” That night, everyone on the ship heard a voice in their sleep calling out: “Oh people of the ship! Abū Dāwūd has purchased his Jannah from Allāh 
for one dirham.”
Imām Abū Dāwūd (raḥimahullāh) passed away on Friday, 16 Shawwal 275 AH, at the age of 73. He was buried in Basrah alongside Sufyān at-Thawrī.
Teachers and Students
He studied under around 300 great scholars of his time. Some of his famous teachers include Imām Aḥmad ibn Ḥanbal, Musaddad ibn Musarhad, Yaḥya ibn Ma`īn, Qutaibah ibn Sa`īd, Abū Bakr ibn Abī Shaibah, `Uthmān ibn Abī Shaibah, `Ali ibn al-Madīnī, `Abdullāh ibn Maslamah al-Qa`nabī, Sulaymān ibn Ḥarb, Abul Walīd aṭ-Ṭayālisī, Isḥāq ibn Ibrāhīm, Muslim ibn Ibrāhīm, and Abū Ja`far an-Nufailī.
He had numerous students from around the world. Among his more noted students are his son Abū Bakr `Abdullāh ibn Abī Dāwūd, Imām at-Tirmidhī, Imām an-Nasā’ī, Abū `Alī Muḥammad ibn Aḥmad al-Lu’lu’ī, Abū Sa`īd Aḥmad ibn Muḥammad ibn al-A`rābī, Abū Bakr Muḥammad ibn Bakr ibn Dāsah, Abū `Īsa Isḥāq ibn Mūsa ar-Ramlī, Abū Bakr ibn Abi ad-Dunyā, Abu at-Tīb Aḥmad ibn Ibrāhīm ibn al-Ashnānī al-Baghdādī, Abu al-Ḥasan `Alī ibn al-Ḥasan ibn al-`Abd al-Anṣārī, Abū `Amr Aḥmad ibn `Alī ibn al-Ḥasan al-Baṣarī, and Imām Aḥmad ibn Ḥanbal. Imām Aḥmad was his teacher, but has also narrated one ḥadīth from him, thus making him his student as well.
The Sunan
When Imām Abū Dāwūd compiled his Sunan, Ibrāhīm al-Ḥarbi stated: “Ḥadīth was made supple (easy) for Abū Dāwūd just as ḥadīd (steel) was made supple for the Prophet Dāwūd 
.” When the Sunan was read out to Ibn al-A`rābī, he commented: “If a man had nothing with him except for the Book of Allāh, and this book (of Abū Dāwūd), he would need absolutely nothing else to go along with them.”
In compiling the Sunan, Imām Abū Dāwūd included only those aḥādīth that fall under topics of Fiqh. He included only one or two aḥādīth under each chapter to make the collection concise and easy to derive fiqh.
He occasionally summarized long aḥādīth for brevity, so that the fiqh didn't get lost in the long narration. In his Sunan, he included several aḥādīth that are mursal in instances where he did not know of a contradictory ḥadīth with a connected chain. He did not include aḥādīth from those considered matrūk al-ḥādīth (a narrator whose aḥādīth are rejected). He included a munkar (mistaken or rejected) ḥādīth only when there were no other ḥādīth in that topic. In such cases, he clearly mentioned that the ḥādīth is munkar. He also included some weak aḥādīth in his Sunan, clearly mentioning their weakness. Al Ḥāfiẓ `Abdullāh ibn Mandah said: “Abū Dāwūd narrated weak chains of narration when he did not find anything else for the topic, because to him, weak aḥādīth were stronger than the opinions of men.” For more information on his methodology in compiling his Sunan, see his letter to the people of Makkah.
According to his student, Ibn Dāsah, Imām Abū Dāwūd said: “I have written 500,000 aḥādīth of Allāh’s Messenger (saws), from which I selected 4800 of the most authentic aḥādīth for this book (the Sunan). Of these, four aḥādīth are sufficient for preserving one’s Deen: ‘Actions are judged according to intentions.’ ‘Of the beauty of a person’s Islam is his leaving out that which does not concern him.’ ‘None of you can be a perfect believer until he loves for his brother that which he loves for himself.’ ‘The halal is clear and the haram is clear, and between these two are unclear matters. Whoever saves himself from these has saved his Deen.’”