He is Ahmed b. Abdillāh Abu Nu‘aym al-Hāfidh al-Asbahāni al-Shāfi‘ī.
Ibn Khallikān said: Asfahāni with the bā replaced with fā is also correct.
He was Persian, born 336h in what is modern-day Iran, son of an ‘Ālim and a Muhaddith, ‘Abdullāh b. Ahmed. He studied under the many scholars present in Persia at that time and then travelled to Makkah, Basrah, Kūfah and Naysabūr. He took from Abu ‘Ali al-Sawwāf and Abu Bakr ibn al-Haytham al-Anbāri in Baghdād, Abu Bakr al-Ājūri in Makkah as well as Fārūq b. ‘Abd’l-Karīm al-Khattābi in Basrah.
His main teachers include the great Imām and Muhaddith Abu al-Qāsim al-Tabarāni, from whom he took many Hadīth, and also Abu Ahmed al-‘Assāl, the Hāfidh and Qādhi of Asbahān. Abu Nu‘aym also studied under the leader of the Muhaddithīn, Shaykh’l-Islām and author of the magnificent “‘Ilal”, Imām al-Dāraqutni, may Allāh have mercy upon them all.
He also had many students, some travelling great distances to take Hadīth from him. The greatest of these students was undoubtedly the Muhaddith of the East, Abu Bakr al-Khatīb al-Baghdādi.
The scholars have agreed on his excellence and status as an Imām, a Faqīh and his status as a Muhaddith. He was also an expert in the Qirā‘āt as mentioned by Ibn al-Jawzi. He is also considered as one of Islam’s foremost Historians, with the “Hilyah” and “Tārīkh Asbahān”.
Al-Khatīb al-Baghdādi said, “I only saw two people that deserved the title Hāfidh: Abu Nu‘aym al-Asbahāni and Abu Hāzim al-‘Abduwi al-A‘raj.”
Al-Subki said about him, “al-Imām al-Jalīl, al-Hāfidh al-Sūfi, the one who combined between fiqh and tasawwuf, the pinnacle of preservation and accuracy…”
Hamza ibn al-‘Abbās al-‘Alawi said, “The people of Hadīth said, “Abu Nu‘aym was unrivalled in Hadīth for 14 years; none could be found in the East or West who had more preserved than him and with a better chain…””
Al-Dhahabi referred to him as Shaykh’l-Islām in “Siyar” and also said, “al-Hāfidh al-Kabīr, the Muhaddith of his time…”
Ibn ‘Asākir said, “The Shaykh, Imām, al-Hāfidh, the best in his time with respect to his excellence, his collections and his knowledge; he authored famous pieces of work, was known to all and many benefitted from his books due to their quality.”
Ibn Kathīr said, “al-Hāfidh al-Kabīr, author of many, famous beneficial works, from them “Hilyat’l-Awliyā’” in a large number of volumes, indicating his great knowledge of narrations and high number of scholars he took from…”
He was criticised by some scholars for the many weak and even fabricated Hadīth that Abu Nu‘aym narrated such as by Ibn Taymiyyah, yet he still said about Abu Nu‘aym, “Hāfidh, Thiqah, Narrator of many Hadīth, broad in his knowledge of narrations…”
Ibn Taymiyyah also said, “He is from the greatest of the memorisers of Hadīth, of the most prolific authors, with the people benefitting from his works; it is not enough to say that he was just thiqah, for he was a level above that…”
Abu Nu‘aym was involved in the normal controversies of his time due to his Ash‘ari leanings, harshly criticised by some of the Hanbali scholars of his time such ibn Mandah and later ibn al-Jawzi (to which he also responded in a harsh way), but he was still respected for his status and as al-Dhahabi mentioned, such criticisms of contemporaries for one another due to differences in creed are not normally used against a scholar. Indeed al-Dhahabi in conclusion praised Abu Nu‘aym describing him as one of the Imams yet said that he spoke against Ibn Mandah “according to his desires” and that he “narrated fabricated Hadīth without indicating such.”
Also, Imām al-Dhahabi (in al-‘Uluw, 1305), Ibn Taymiyyah (in al-Majmū‘, 60/5) and Ibn al-Qayyim (in Ijtimā‘ al-Juyūsh’l-Islāmiyyah, 279) went to great lengths defending Abu Nu‘aym and his ‘aqīdah against some of the more critical Hanbali scholars of his time (such as ibn al-Jawzi), affirming for him the ‘aqīdah of Ahl’l-Sunnah.
Abu Nu‘aym wrote over 50 books, the most famous and beneficial of them the collection of sayings and narrations of many of the early Muslims in his book “Hilyat’l-Awliyā’ wa Tabaqāt’l-Asfiyā’”, starting off with the ten promised Paradise, then the Ahl’l-Sufah, then the Ascetics of the Companions, then the Tābi‘īn and their followers and then some of the Imāms, Scholars, Sūfis and Awliyā’ after them, in no particular order of name, age, place of birth or excellence, as is usually the case in other historical works.
Ibn Taymiyyah said, “His book the Hilyah is from the best works of the later historians with its narrations of the ascetics…”
He passed away at the age of 94 in the year 430h, rahimullāh.
Sources: Siyar, Tabaqāt’l-Shāfi‘iyyah 18/4, Tadhkirat’l-Huffādh 1092, al-Ahādīth al-Mu‘allah fi Kitāb’l-Hilyah 4/1