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.