The four Imam's - Imam Abu Hanifa, Imam Malik, Imam Ahmed bin Hanbal and Imam Shafi - are the main figures who's interpretation of the Hadith and Qur’an are followed by the majority of Sunni Muslims all over the world. They are therefore of great importance to the correct practice of Islam.
Imaam Ahmad ibn Mohamed Ibn Hanbal, Abu `Abd Allah al-Dhuhli Al-Shaybani al-Marwazi al-Baghdadi (d. 241).
Imam Ahmed ibn Hanbal (780 - 855) was among the great Muslim scholars and theologians. He is the founder of the Hanbali school of fiqh. His full name was Ahmed bin Muhammad Hanbal.
Imam Hanbal was born in Central Asia to Arab parents in 780. After his father died, he moved to Iraq and studied extensively in Baghdad, and later used his travels for further education. He was mainly interested in the science of Hadith and traveled extensively through Iraq, Syria, and Arabia studying religion and collecting traditions of Prophet Mohammad (PBUH).
His travels lasted several years. Upon returning home, he studied under Imam Shafi on Islamic law. Imam Hambal was very devoted to traditional views and was opposed to innovations in Islamic law.
Imam ibn Hanbal became very well known all, and his teachings spread worldwide. His learning, piety and unswerving faithfulness to traditions gathered a lot if admirers around him.
From the biographical notice on Imam Ahmad in the Reliance of the Traveler:
"Out of piety, Imam Ahmad never gave a formal legal opinion (fatwa) while Imam Shafi`i was in Iraq, and when he later formulated his school of jurisprudence, he mainly drew on clear texts from the holy [Qur’an], Hadith, and scholarly agreement, with relatively little expansion from analogical reasoning (qiyâs). He was probably the most learned in the sciences of Hadith of the four great Imams of Sacred Law; Imam Abu Hanifah, Imam Al Shafi`i`, Imam Malik, and him Imam Ahmad Ibn Hanbal.
His students included many of the foremost scholars of Hadith. Abu Dawud said of him: ‘Ahmad’s gatherings were gatherings of the afterlife: nothing of this world was mentioned. Never once did I hear him mention this-worldly things.’
... Imam Ahmad never once missed praying in the night, and used to recite the entire [Qur’an] daily. Imam Ahmad was imprisoned and subjected to various forms of torture for twenty-eight months under the Abbasid caliph al-Mu`tasim in an effort to force him to publicly support the [Mu`tazila] position that the Holy [Qur’an] was created, but the Imam refused to give up the belief that the [Qur’an] is the uncreated word of Allah, after which Allah delivered and vindicated him.
… When Imam Ahmad died in 241/855, he was accompanied to his resting place by a funeral procession of eight hundred thousand men and sixty thousand women, marking the departure of the last of the four great mujtahid Imams of Islam."
Al-Dhahabi describes him saying:
"The true Sheikh of Islam and leader of the Muslims at his time, the Hadith master and proof of the Religion. He took Hadith from Hushaym, Ibrahim ibn Sa`d, Sufyan ibn `Uyayna, `Abbad ibn `Abbad, Yahya Ibn Abi Za’ida, and their layer. From him narrated al-Bukhari [two Hadiths in the Sahih], Muslim , Abu Dawud , Abu Zur`a, Mutayyan, `Abd Allah ibn Ahmad, Abu al-Qasim al-Baghawi, and a huge array of scholars. His father was a soldier one of those who called to Islam and he died young."- Al-Dhahabi continues.
Ibn al-Jawzi relates in al-Manaaqib (p. 192):
"Imaam Ahmad was the foremost among the Imams in collecting the Sunnah and sticking to it."
`Abd Allah ibn Ahmad said:
"I heard Abu Zur`a [al-Razi] say: ‘Your father had memorized a million Hadiths, which I rehearsed with him according to topic.’"
"I heard Abu `Abd Allah say: ‘I memorized everything which I heard from Hushaym when he was alive.’"
Ibrahim al-Harbi said:
"I held Ahmad as one for whom Allah had gathered up the combined knowledge of the first and the last."
Imam al-Shafi`i said:
"You (addressing Imaam Ahmad ibn Hanbal) are more knowledgeable about Hadith than I, so when a hadith is sahih (correct), inform me of it, whether it is from Kufah, Basrah or Syria, so that I may take the view of the hadith, as long as it is sahih.
Related by Ibn Abi Haatim in Aadaab ash-Shaafi'i (pp. 94-5),
"I heard Imaam al-Shafi`i say: ‘I left Baghdad and did not leave behind me anyone more virtuous (afdal), more learned (a`lam), more knowledgeable (afqah) than Ahmad ibn Hanbal.’"
`Ali ibn al-Madini said about Imam Ahmed Ibn Hanbal:
"Truly, Allah reinforced this Religion with Abu Bakr al-Siddiq the day of the Great Apostasy (al-Ridda), and He reinforced it with Ahmad ibn Hanbal the day of the Inquisition (al-Mihna)."
Abu `Ubayd said:
"The Science at its peak is in the custody of four men, of whom Ahmad ibn Hanbal is the most knowledgeable."
Ibn Ma`in said, as related by `Abbas [al-Duri]:
"They meant for me to be like Ahmad, but by Allah! I shall never in my life compare to him."
A lot of great scholars who followed the approach (Madhab) of Imam Ahmad Ibn Hanbal.
Ibn ul-Qayyim al-Jawziyyah says in his book Imam Bukhari, Imaam Muslim and Imaam Abu Dawood were strong followers of Imam Ahmad Ibn Hambal. Among other great Imams who followed the teachings of Imam Ahmad include Shaikh-ul-Islam Taqi ad-Deen Ibn Taymiyyah, Abdul Qadir Jillaani, Ibn ul-Qayyim al-Jawziyyah, Al-Haafidh Zaynud-Deen Ibn Rajab al-Hanbali, Imam Muhammed Bin Abdul Wahab and Imam Ibn ul Jawzi.
Ibn al-Jawzi narrates from Bilal al-Khawass that the latter met al-Khidr and asked him: "What do you think of al-Shafi`i?" He said: "One of the Pillar-Saints (Awtâd)." " What about Ahmad Ibn Hanbal?" "He is a Siddiq."
Ibn al-Jawzi also narrates that Imam Ahmad ibn Hanbal said:
"Whoever rejects a statement of Prophet Mohammad (PBUH) is on the brink of destruction." and was the foremost in sticking to the Sunna and sahih hadith.
Among Imam ibn Hanbal’s works is the great encyclopedia of Traditions called Musnad, collected by his son from his lectures and amplified by supplements - containing over twenty eight thousand traditions. Other works include Kitab-us-Salat, on the Discipline of Prayer and Kitab-us-Sunnah, on the Traditions of Prophet Mohammad (PBUH).
Main sources: al-Dhahabi, Siyar A`lam al-Nubala’ 9:434-547 #1876 and Tadhkira al-Huffaz 2:431 #438.