It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Knowledge is three things, and whatever is besides them is extra: a decisive verse, an established Sunnah, and a just obligation."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13669