These are brief biographies of the seven great imams of hadīth:

1. AHMAD (Ibn Hanbal):

He is Abu ‘Abdullah, Ahmad bin Muhammad bin Hanbal Ash-Shaibāni, known by the name Ibn Hanbal.

He was a celebrated theologian, jurist and a Hadīth scholar; one of the four Fiqh Imāms and the founder of what later came to be known as Madhhab Al-Hanbaliya (The Hanbal’s Juristic school).

Ahmad was born in Baghdad in the month of Rabi'ul-Awwal, 164 H. He studied the Hadīth and Fiqh together with other Islamic disciplines in Baghdad, then traveled to Ash-Shām and Hijāz for further studies. 

He was persecuted during the rule of Al-Mā'mūn bin Hārun Ar-Rashid for refusing to acknowledge the Bid'a of claiming the creation of the Qur'ān introduced by the Mu'tazila.

He, however, stood firm against all the trials and saved the Sunnah from the innovation of the wretched Mu'tazila thoughts. He was the mostly persecuted and most firm amongst all the Imāms.

Ahmad is most famous for collecting the Ahadīth of the Prophet ﷺ compiled in the Musnad Ahmad bin Hanbal, which contains 28 to 29 thousand Ahadīth. It was said that Ibn Hanbal memorized one million (1,000,000) Ahadīth.

Ibn Hanbal died in Baghdad on Friday, 12 Rabi’-ul-Awwal 241 H. 

2. AL-BUKHĀRI, Muhammad bin Ismā'il:

He is the Amir-ul-Mu'minīn in the knowledge of Hadith, and his full name is Abu 'Abdullah, Muhammad bin Ismā'il bin Ibrāhim bin Al-Mughira bin Bardizah Al-Ju'fi Al Bukhāri. 

He was born in Shawwāl 194 H. in Bukhāra in what is now Uzbekistan. Al-Bukhāri studied Ahadīth at an early age and traveled widely around the Muslim world collecting Ahadīth and compiled the most authentic ones in his book Al-Jāmi As-Sahih, which later became known as Sahih Al-Bukhāri and contains 2602 Ahadīth which he selected from the thousands of Ahadīth that he had memorized.

Al-Bukhāri studied under many famous Hadīth Imams like 'Ali bin Al-Madini and Yahya bin Ma'in. He died in Samarqand, the present day capital of Uzbekistan, the night of 'Eid-ul-Fitr, 256 H.

Al Bukhāri was unique in the science of the methodology of Hadīth and his book Al-Jāmi As-Sahih is considered to be the most authentic book after the Qur'ān.

3. MUSLIM bin Hajjāj:

Muslim’s full name is Muslim bin Al-Hajjāj Al-Qushairi An-NIshapuri. He was born in 204 H, in the city of Nishapur near the city of Mashhad in present Iran.

Muslim is considered second only to Al-Bukhāri in the science of the methodology of Hadīth. He started the study of Hadīth at an early age and traveled to Iraq, Hijāz, As-Shām and Egypt and studied under the scholars of Hadīth at the time like Al-Bukhāri, Ahmad bin Hanbal and Ibn Abu Shaiba. He also taught the famous Hadīth scholars like At-Tirmidhī and Ibn Abu Hātim.

Muslim compiled the Hadīth book Al-Musnad As-Sahih, which became known as Sahih Muslim. This book, which is considered by the Muslim 'Ulama as the second most authentic Hadīth book after Al-Bukhāri, contains 9,200 Ahadīth.

Imam Muslim died at his birthplace in Rajab 261 H.

4. ABU DĀ'UD, Sulaiman bin Al-Ash'ath:

He is Abu Dā'ud, Sulaiman bin Al-Ash'ath bin Ishāq Al-Azdi As-Sijistāni. He was born in 202 H.

He studied Ahadīth under Imām Ahmad bin Hanbal along with Al-Bukhāri and taught many of the later scholars of Hadīth, like At-Tirmidhī and An-Nasā'i.

Though Abu Dā'ud collected 500,000 Ahadīth, he only compiled 4,800 Ahadīth in his book As-Sunan, which he taught in Baghdad and other major cities at that time. 

He died at Basra on Friday in the month of Shawwāl 275 H.

5. AT-TIRMIDHĪ, Abu 'Isa Muhammad bin 'Isa:

Abu 'Isa, Muhammad bin 'Isa bin Sura At-Tirmidhī was born in 209 H. in a town called Tirmidh in Uzbekistan near the northern border of Afghanistan.

He was a student of Al-Bukhāri, and compiled 4,000 Ahadīth in his book called Al-Jāmi which later o be known as Sunan At-Tirmidhī. He also contributed trememndously to the methodology of Hadīth and composed a book on it called Al-'Ilal (the discrepancies). He was famous for his piousness and asceticism. 

He became blind and finally died on 13 Rajab 279 H.

6. AN-NASĀ'I, Abu 'Abdur-Rahmān Ahmad bin Shu'aib:

Abu 'Abdur-Rahmān, Ahmad bin 'Ali bin Shu'aib bin 'Ali Al-Hāfiz was born in 215 H. in Nisa’, a city in Khurāsan. 

He became famous for the study of the methodology of Hadīth, memorizing and mastering it. His book known as Sunan An-Nasā'i is third to Sahih Bukhāri interms of containing the least weak Ahadīth.

He lived in Egypt then moved to Damascus, Syria and died in Makkah in the year 303 H.

7. IBN MĀJAH, Muhammad bin Yazid:

Abu 'Abdullah, Muhammad bin Yazid bin Mājah Al-Qizwini was born in 207 H.

He studied under many scholars of Hadīth, and many people narrated Ahadīth from him. He was one of the eminent scholars of Hadīth, but his Sunan contains many weak and even Munkar Ahadīth.

Ibn Mājah died in Ramadān in 273 H. 

Bulugh Al-Marâm | بلوغ المرام
(Attainment of the objective)
according to the evidence of the ordinances 

Compiled by:
Al-Hāfiz Ibn Hajar Al-Asqalani
pp. 11-12