Imam Nawawi (d.676H) said
“Scholars have compiled forty hadith on the foundations of the religion, some on the subsidiary matters, some on jihad, some on abstinence, some on etiquettes and others on sermons. All of these are righteous aims – may Allah be pleased with those who pursued them.
However, I considered it best to compile forty hadith, which are more important than that, as these forty hadith incorporate all of these [individual] subjects. The scholars described these hadith as being the axis of Islam, half of Islam,or a third of it, and so forth.
I will only include authentic ahadith in this compilation of forty hadith. The majority of them are found in the Sahih of Al=Bukhari and Sahih Muslim. I cite them without the chain of narrations, so as to make it easier to memorise them and to make them as beneficial as possible – if Allah, the Most High wills. I also included a section explaining the meanings of any obscure words.
Every person who desires and yearns for the hereafter should know, these ahadith because of the important matters they contain and the direction they give in respect to all forms of obedience. This is clear to anyone who has reflected upon them. I rely solely upon Allah and entrust my affairs only to Him. To Him is all praise and grace, and with Him is success and immunity from error.”