Imam An-Nawawi | Hikmah Publications
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 that 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.