Here are forty hadiths covering the most important aspects of religion. Each of these hadiths represents one of the great foundations of the religion and is one of those that the ulema called the core of Islam as a whole, or its half, or its third, etc. In addition, the author adhered to the rule according to which each of these forty hadiths must be authentic, and most of them must be cited in both the Sahih of al-Bukhari and the Sahih of Muslim.