Abd Allah ibn Ahmad (Imam Ahmad ibn Hanbal's son) said: "I heard Abu Zur`a [al-Razi] say: ‘Your father had memorized a million hadiths, which I rehearsed with him according to topic."
Ibrahim al-Harbi said: "I held Ahmad as one for whom Allah had gathered up the combined knowledge of the first and the last."
Harmala said: "I heard al-Imam al-Shafi`i say: ‘I left Baghdad and did not leave behind me anyone more virtuous (afdal), more learned (a`lam), or more knowledgeable (afqah) than Ahmad ibn Hanbal.’"
`Ali ibn al-Madini said: "Truly, Allah reinforced this Religion with Abu Bakr al-Siddiq the day of the Great Apostasy (al-Riddah), and He reinforced it with Ahmad ibn Hanbal the day of the Inquisition (al-Mihna)." This is referrring to the lies that were said about the Quran being a creation of Allah (SWT).
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."
Muhammad ibn Hammad al-Taharani said: "I heard Abu Thawr say: ‘Ahmad is more learned and more knowledgeable than al-Thawri."
Al-Dhahabi concludes: "Al-Bayhaqi wrote Abu `Abd Allah’s biography (sîra) in one volume, so did Ibn al-Jawzi, and also Shaykh al-Islam [`Abd Allah al-Harawi] al-Ansari in a brief volume.
"He passed on to Allah’s good pleasure on the day of Jum`a, the twelfth of Rabi` al-Awwal in the year 241, at the age of seventy-seven. I have two of his short-chained narrations (`awâlîh), and a licence (ijâza) for the entire Musnad."
Al-Dhahabi’s chapter on Imam Ahmad in Siyar A`lam al-Nubala’ counts no less than 113 pages.
Imam Ahmad ibn Hanbal had many teachers, the most prominent was Imam Shafiee (RA) whom he met on several separate occasions and each time he took full benefit of the opportunity. Imam Ahmad ibn Hanbal commented, ‘Only after sitting in the Majlis of Imam Shafiee (RA) did I understand and comprehend nasikh and mansukh hadith.’ even though by this time he was very knowledgeable in uloom-al-hadith.
It was only at the age of forty, in 204 A.H., the Imam began formally teaching hadith. Whilst his teachers were still alive he refused to teach and narrate hadith out of humility and respect for them. Imam Ahmad was acknowledged by the Ulama of his time as Imam ul Hadith. May Allah (SWT) have mercy on him.