بسم الله الرحمن الرحيم
السلام عليكم و رحمة الله و بركاته
Sheikh Abdur-Razaaq Al-Badr Al-'Abaad may Allah preserve him said in his explanation of the 40 ahadeeths of Al-Imaam An-Nawawee rahimahu Allaah:
"The second of the 40 ahadeeths ( in reality 42) is called by some of the scholars " Ummu As-Sunnah" because it gathers the details of the Sunnah;rather it gathers all the religion with its three different levels: Al-Islaam, Al-Eemaan, Al-Ihsaan. So the hadeeth is general and the rest of the Sunnah has detailed it as the Faatiha (Ummu Al-Quraan) is general and has been detailed by the rest of the Quraan.
http://www.al-badr.net/web/index.php...ry&category=63