[The Shi'ah Encyclopedia]
 

Evidence, Analysis and Conclusion

Sahih al-Bukhari is the foremost book of hadith for the Ahl al-Sunnah and is considered by them to be the most reliable book after the Qur'an.  Its English translation was undertaken by Muhammad Muhsin Khan in 9 volumes and has been published in many editions.

The hadith under discussion appears in the Dar al-Fikr edition (which is non-dated, though apparently recent) as follows:
 

Sahih al-Bukhari, Al-Bukhari, Muhammad b. Isma`il (d. 256 AH), volume 4, pg. 437, hadith number 658 Beirut: Dar al-Fikr (9 vols), translated by Muhammad Muhsin Khan, n.d. [Harvard] [London] [Ohio State] [Toronto] [Library of Congress]

Those with even a rudimentary understanding of Arabic text will be able to see that the underlined arabic text of the hadith (wa imaamukum minkum) and the English equivalent are completely different! 

An accurate translation would be:

This translation can be seen in James Robson's translation of Khatib al-Tabrizi's Mishkat al-Masabih, which quotes the hadith from Sahih al-Bukhari:
 

Mishkat al-Masabih, al-Khatib al-Tabrizi (d. 737 AH), volume 3, pg. 1159, chapter vi (The Descent of Jesus) Lahore: Shaikh Muhammad Ashraf (2 vols), translated by James Robson, 1964  [Harvard] [London] [Toronto] [Ohio State] [Library of Congress]

Perhaps this is an innocent mistake by the translator of Sahih al-Bukhari?

The translation by Muhammad Muhsin Khan was checked and re-checked by several scholars, as can be seen from the 'certificate' on the first page of each volume:
 

Sahih al-Bukhari, Al-Bukhari, Muhammad b. Isma`il (d. 256 AH), volume 4, pg. 1 Beirut: Dar al-Fikr (9 vols), translated by Muhammad Muhsin Khan, n.d. [Harvard] [London] [Ohio State] [Toronto] [Library of Congress]

So who is responsible for this error?

This 'certificate' also appears in an earlier edition published in Pakistan in 1971. 

In that edition we find that the blatant 'error' does not occur!

The hadith is translated more accurately.  Notice the volume, page, and hadith numbers in the edition below, which are identical to the Dar al-Fikr edition mentioned above.
 

Sahih al-Bukhari, Al-Bukhari, Muhammad b. Isma`il (d. 256 AH), volume 4, page 437, hadith number 658 Pakistan: Sethi Straw Board Mills (Conversion) Ltd (9 vols), translated by Muhammad Muhsin Khan, 1971 CE [Harvard] [London] [Ohio State] [Toronto]

So it seems that this 'error' is actually a clear and deliberate tampering with the text of the translation.  It crept into later editions which are reprinted even now and still reflect this distortion (tahrif).  This includes the later editions printed in Pakistan.  Even the online hadith databases have the wrong translation.  For example, see:
 

Sahih al-Bukhari, Al-Bukhari, Muhammad b. Isma`il (d. 256 AH), volume 4, book 55, hadith number 658 Online translations: [University of Southern California MSA] [University of Northumbria Islamic Society]

What is the reference to 'Fateh ul-Bari' in the distorted versions?

Fath al-Bari is the most famous commentary (sharh) on Sahih al-Bukhari.  It was written by Ibn Hajar al-'Asqalani (d. 852 AH) a scholar of great repute amongst the Ahl al-Sunnah.  Although the analysis above demonstrates the clear and deliberate tampering of the text, an investigation into the commentary on this hadith in Fath al-Bari is even more revealing.  It is a reasonably lengthy text where Ibn Hajar quotes the opinions of various authorities regarding the meaning and import of this tradition.  Some of the comments are translated below:
 

Fath al-Bari bi Sharh Sahih al-Bukhari, Ibn Hajar al-'Asqalani (d. 852 AH), under hadith number 3193 [Online Fath al-Bari at www.al-islam.com] (requires arabic support) [Downloadable Fath al-Bari at muhaddith.org] (requires arabic support)     And according to Ahmad within the hadith of al-Dajjal and descent of 'Isa "If they were with Isa, it would be said: Lead O' Ruh Allah (i.e. 'Isa), he would say: Let your imam lead you in prayer." Also Ibn Majah in the long hadith of Abi Umamah about al-Dajjal, he said: and all of them, meaning the Muslims, in Jurusalem and their imam is a righteous man who has advanced to lead them in prayer, when 'Isa has descended; the imam steps back to allow Isa to lead. Isa would stand between his shoulders [i.e. facing him] then would say: Lead! The [prayer] was setup for you. Abu al-Hasan al-Khasa'i al-'Abidi said in Manaqib al-Shafi'i that the reports are repeated (mutawatir) that al-Mahdi is from this nation and that 'Isa prays behind him. He made mention of this refuting the hadith that Ibn Majah came out with from Anas which has in it "no Mahdi except 'Isa". And Abu Dharr al-Harawi said that he was told by al-Jawzaqi from some of the earlier [men], he said: the meaning of his saying "your imam is from amongst you" means that he rules by the Quran and not by the Gospel. [...] Ibn al-Jawzi said: If 'Isa were to lead there would be a doubt in peoples minds on whether he would be leading as a representative or as an initiator of [new] law. Therefore, he would pray while being lead so that he is not clouded with doubt, from the point of the saying [of  our prophet] “no prophet after me.” Also, by way of 'Isa’s prayer behind a man from this nation, happening at the end of time and near the Hour, is a correct proof to the saying that the Earth is not devoid of a standing proof of Allah (qa'im lillah bi-hujjah), and Allah knows best.

It becomes apparent from the above extract that various explanations were quoted by Ibn Hajar to throw light on the meaning of this hadith and the identity of the imam.  The tampering of the text of the Muhsin Khan translation was achieved by replacing the translation with one of the several explanations, the one colored red.  All the others including the ones above colored blue were ignored.

So who is the "imam" mentioned in the tradition?

The Shi'ah understand this to be a reference to Imam al-Mahdi (a) who is the Twelfth Imam and Successor to the Prophet (s) from his Family (Ahl al-bayt).   He is the righteous Qa'im al-Hujjah, behind whom Jesus (a) will pray when he descends.  See for details:

• Imam Mahdi - Just Leader of Humanity
• The Awaited Saviour
• Shi'ah Encyclopedia
• Also see the heading "Ahlul Bayt - Imam Zayn al-'Abidin [a] to Imam

 Mahdi [a]" in the Subject Index