Refutation of one who casts aspersions upon Saheeh al-Bukhaari
Question

My Shia friend have posed a question to me as to how we can claim Sahih Bkhari to be Authentic when Imam Bukhari (May Allah Mercy be upon him) was present 400 years after the death of the Prophet.
Praise be to Allah.Imam al-Bukhaari (may Allaah have mercy on him) died in 256 AH, i.e., 245 years after the Prophet (peace and blessings of Allaah be upon him) died, not as your Shi’i friend claims. But these people are natural liars so this claim comes as no surprise. That does not mean that al-Bukhaari could have narrated directly from the Prophet (peace and blessings of Allaah be upon him); this is not what is meant at all. Rather we mention this here for the purpose of making things clear. 
But how can we depend upon Saheeh al-Bukhaari when al-Bukhaari never met the Prophet (peace and blessings of Allaah be upon him) in person? 
The answer is that in his Saheeh, al-Bukhaari did not narrate anything directly from the Prophet (peace and blessings of Allaah be upon him), rather he narrated from trustworthy shaykhs, who attained the highest degree of memorization, precision and trustworthiness, who in turn had narrated from equally reliable shaykhs, all the way back to the Sahaabah who narrated from the Messenger of Allaah (peace and blessings of Allaah be upon him). The smallest number of narrators between al-Bukhaari and the Prophet (peace and blessings of Allaah be upon him) is three. So we rely upon Saheeh al-Bukhaari because he chose the narrators from whom he transmitted hadeeth with the utmost care, and they are the most trustworthy. Moreover, he would not write down any hadeeth in his Saheeh until he had done ghusl, then prayed two rak’ahs and prayed istikhaarah, asking Allaah to guide him with regard to writing down this hadeeth; then he would write it down. It took him sixteen years to write this book, which the ummah accepted and unanimously agreed that what is narrated in it is saheeh; and Allaah has protected this ummah from agreeing upon misguidance. 
Imam al-Nawawi (may Allaah have mercy on him) said in his introduction to Sharh Muslim (1/14): “The scholars (may Allaah have mercy on them) are agreed that the most sound of books after the Qur’aan are the two Saheehs of al-Bukhaari and Muslim, which were accepted by the ummah. The book of al-Bukhaari is the more sound and the more beneficial of the two.” 
If you ask this Shi’i (or Raafidi) about the sayings of ‘Ali (may Allaah be pleased with him) that their leaders have transmitted, and the sayings of al-Baaqir and Ja’far al-Saadiq, and others of Ahl al-Bayt (may Allaah have mercy on them), whether they heard them from them or transmitted them with isnaads (chains of narrators), the answer will be clear. There is a great difference between the isnaads of al-Bukhaari and the isnaads of these misguided people who cannot find the names of their chain of narrators in the reports except in the books of the weak narrators, liars and deficient narrators. 
The claim made by this Raafidi (Shi’i) is simply the first step in attacking the Sunnah which demonstrates that their madhhab is false and their beliefs are corrupt. They cannot help but try to stir up trouble with these misguided notions. But they can never succeed, for the truth is clear and falsehood is confused.
 Moreover we advise you – may Allaah help you – to look for Sunni friends from whose company you can benefit, and to avoid making friends with followers of bid’ah. The scholars have warned against making friends with them because they will keep on trying until they take a person far away from the truth by using all kinds of wiles and causing confusion.
 We ask Allaah to help us and you to follow the Sunnah and to keep away from bid’ah and those who follow it.
 And Allaah knows best.