It is reported from Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Knowledge is three, and whatever is besides that is extra: a decisive verse, an established Sunnah, and the obligatory shares by which the estate can be divided justly."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12172