Yusuf bin Musa (rahimahullah) narrates the following incident:
On one occasion, I was in the Jaami’ Musjid of Basrah when a person called out announcing, “O Ulamaa! Muhammed bin Ismaa’eel Bukhaari has arrived!” Hearing this, I, together with a few other Ulamaa, immediately stood and went to meet him. On seeing him, we found him to be a young person, performing salaah behind a pillar in the musjid.
As soon as he completed his salaah, the Ulamaa thronged around him and requested him to hold a sitting in which he would narrate Ahaadith to them. Imaam Bukhaari (rahimahullah) accepted their request and agreed to narrate to them the following day.
The next day, several thousand Ulamaa of Basrah gathered in anticipation of the Hadith lesson of Imaam Bukhaari (rahimahullah). When Imaam Bukhaari (rahimahullah) arrived in the gathering and sat, he addressed the people and said, “O people of Basrah! I am still young; and therefore I feel difficult to narrate to you. However, since you have requested me to narrate to you, I will do so. Nevertheless, I will narrate Ahaadith to you from the narrators of your city, Basrah, so that you may receive more benefit.”
Imaam Bukhaari (rahimahullah) then began to narrate saying, “Abdullah bin Uthmaan bin Jibillah bin Abi Rawaad reports from his father, who reports from Shu’bah, who reports from Mansoor and others, who reports from Saalim bin Abil Ja’d, who reports from Hazrat Anas (radhiyallahu ‘anhu), who reports that on one occasion, a Bedouin came to Rasulullah (sallallahu ‘alaihi wasallam) and said, ‘O Rasul of Allah (sallallahu ‘alaihi wasallam)! A man loves certain people…’” Imaam Bukhaari (rahimahullah) then narrated the entire Hadith to them.
After narrating the Hadith, Imaam Bukhaari (rahimahullah) said to the people of Basrah, “You do not have this chain for this Hadith. Rather, the chain which you have for this Hadith, O people of Basrah, is via another student of Saalim, besides Mansoor.”
Imaam Bukhaari (rahimahullah) continued to narrate Ahaadith to the people of Basrah in this manner. With every Hadith he narrated, he said, “According to the chains which you have in Basrah, Shu’bah has narrated this Hadith in such-and-such manner. However, this Hadith is also reported via the chain of so-and-so, and this chain is not found among you in Basrah.”
Thus, for every Hadith that Imaam Bukhaari (rahimahullah) narrated to the Ulamaa of Basrah, he presented to them the chain which they had among them in Basrah, and also presented them with the chain that they did not have, showing them the difference between the chains.
From this, it is clear that Imaam Bukhaari (rahimahullah) was blessed with such expertise in Hadith that he even knew which chains of the Ahaadith were commonly found in which areas of the Islamic world.
(Siyar A’laamin Nubalaa 12/409)