1 – Ibn Hajar al-‘Asqalani narrated that Abu Ahmad bin ‘Udayy said:
“I heard a number of the scholars of Baghdad say that when al-Bukhari arrived in Baghdad, the scholars of Hadith heard of this, and gathered together to test his memorization. So, they gathered a hundred ahadith and scrambled their texts and chains, putting the text of this hadith with the chain of a different one, and the text of that hadith with the chain of a different one, etc. They then divided these hundred ahadith between ten individuals, so that each man had ten ahadith. These men were then told to narrate these scrambled ahadith to al-Bukhari when they arrived at the scheduled gathering.
So, they attended the gathering, as did a group of strangers from Khurasan and Baghdad. When the guests were comfortable, one of the ten men turned to al-Bukhari and asked him about one of the ahadith that were with him. So, al-Bukhari said: “I do not recognize it.” The man continued narrating each hadith, one after the other, until he had narrated all ten, with al-Bukhari saying to each one: “I do not recognize it.” The scholars who were present at the gathering began turning to each other in surprise, and whoever did not follow the incident to the end would judge al-Bukhari to be deficient, inept, and of bad memory.
The second of the ten men then turned to al-Bukhari to ask him about another of these scrambled ahadith, to which al-Bukhari said: “I don’t recognize it.” He was asked about another, to which he said: “I don’t recognize it.” The man continued narrating each hadith, one after the other, until he had narrated all ten, with al-Bukhari saying to each one: “I do not recognize it.” The third of the men then turned to al-Bukhari, followed by the fourth man, until all ten men had asked about these hundred scrambled ahadith, with al-Bukhari responding to each of them with nothing more than: “I don’t recognize it.”When he saw that they had narrated all that they had, al-Bukhari turned to the first man, saying: “As for your first hadith, you narrated such-and-such, while what is correct is actually such-and-such. As for your second hadith, you said this, while the correct narration is actually that,” doing the same for the third, fourth, and all the way until he finished with the last man, returning each text to its proper chain, and each chain to its proper text, with all hundred ahadith.”
“And what Hashid bin Isma’il described regarding their days of studying in Basrah together has already been mentioned, in which al-Bukhari would memorize everything that he heard, and never had to write anything down.”
[‘Hadi as-Sari’ (the introduction to ‘Fath al-Bari’); p. 510-511]
2 – Ibn Hajar also narrated that Abu al-Azhar said:
“There were around 400 scholars of Hadith in Samarqand, and they all gathered together to try to cause Muhammad bin Isma’il al-Bukhari to make a mistake. So, in order to test him, they mixed the chains of Sham with the chains of Iraq, and the chains of Iraq with the chains of Sham, and the chains of the Haram with the chains of Yemen.
With all of this, they were unable to cause him to make a single mistake.”
[‘Hadi as-Sari’; p. 511]
3 – Abu Ja’far al-Warraq narrated:
“I asked Abu ‘Abdillah Muhammad bin Isma’il al-Bukhari: “How old were you when you began seeking knowledge of Hadith?”
He replied: “I was inspired to study Hadith while I was still memorizing the Qur’an.”
I asked him: “And how old were you at the time?”
He replied: “Ten years or younger. I finished memorizing it at ten. Then, I went to ad-Dakhili and others to study Hadith. One day, he said, when he was teaching to the people: “Sufyan narrated to us on the authority of Abu az-Zubayr, who narrated to us on the authority of Ibrahim…” So, I said: “O Abu Fulan! Abu az-Zubayr did not narrate on the authority of Ibrahim!” So, he looked at me annoyingly, and I said to him: “Go back to your books and verify this for yourself, if you have them.” So, he went and looked, then he came out and said: “So, what is the correct chain, boy?” I said: “Rather, it is az-Zubayr bin ‘Udayy who narrated from Ibrahim.” So, he took a pen, corrected his notes, and told me: “You are correct.”
Some of those listening to this story asked al-Bukhari: “How old were you when you corrected him like this?”
He answered: “Eleven. When I reached the age of 16, I had memorized all of the books of Ibn al-Mubarak and Waki’. Afterwards, I went out with my mother and brother to Makkah. When I performed the Hajj, my brother returned, and I stayed with my mother studying Hadith. When I was 18, I began to write a book of the narrations of the Sahabah and Tabi’in (now known as ‘at-Tarikh’).””
[‘Sifat as-Safwah’; 2/345]
4 – Sulayman bin Shu’bah said:
“The students of Abu Dawud wrote over 40,000 ahadith from him, and he did not refer to a book to narrate any of them.”
[”Uluww al-Himmah’; p. 183]
5 – Abu Zur’ah said:
“Ahmad bin Hambal had memorized one million ahadith (including those which are weak and authentic).”
[”Uluww al-Himmah’; p. 183]
6 – It was narrated that Shaykh Sulayman al-‘Alwan had completed memorization of the Qur’an by the time he was 18, and had memorized the texts of and studied the explanations of the following: ‘Kitab at-Tawhid,’ ‘al-Wasitiyyah,’ ‘al-Hamawiyyah,’ ‘al-Bayquniyyah,’ ”Umdat al-Ahkam,’ ‘al-Ajrumiyyah,’ ‘Nukhbat al-Fikr,’ ‘ar-Rahbiyyah,’ ‘Bulugh al-Maram,’ ‘al-Usul ath-Thalathah,’ ‘al-Waraqat,’ ‘Mulhat al-I’rab,’ ‘al-Alfiyyah,’ ‘Kashf ash-Shubuhat,’ as well as the six books of Hadith of al-Bukhari, Muslim, at-Tirmidhi, Abu Dawud, an-Nasa’i, and Ibn Majah by the time he was in his twenties!
[‘Imatat al-Litham fi Sirat ash-Shaykh Sulayman al-‘Alwan’]