There were some scholars who collected forty hadiths on the fundamentals, while others did so on derived matters, others on jihad, others on indifference to worldly things, others on conduct, others on speeches, etc., and all of them are sound directions; may Allah be pleased with those who followed them. But I have considered collecting forty more important than all of those, and they would be forty hadiths inclusive of all that, but each hadith would be a great precept from those of the religion - one that scholars had described as being the 'axis' of Islam or 'half' of Islam or 'a third' of it or something similar.
Furthermore, Imam al-Nawawi would commit himself, regarding these forty, to their being authentic, and most of them are found within the two Sahih's of al-Bukhari and Muslim. He cites them with the chains of narration deleted in order to make their memorisation easy and their benefit widespread,
The present selection is generally regarded as the most popular anthology and the best introduction to the study of prophet Muhammad's sayings which together with the Quran contain the essential teachings of Islam.