Allah, The Almighty, has determined the obligatory shares of inheritance Himself – a task which He entrusted neither to a close angel nor to a sent Prophet. He determined the share of each heir in the inheritance, 

which He showed in detail (in His Book), unlike many rulings which were mentioned in a general and brief way in the Quran. In addition, the Sunnah of the Prophet, sallallahu ‘alayhi wa sallam, came to give a detailed explanation about these rulings, just like the rulings regarding the prayer, Zakah (alms) and Hajj

In relation to the obligatory shares of inheritance, the verses of inheritance were revealed in detail as shown in the beginning and end of Surat An-Nisaa’. Allah, The Almighty, calls them His limits, and promises to give a great reward to the one who abides by them and does not transgress them, and threatens to punish the one who transgresses them. He Says (what means):

{These are the limits [set by] Allah, and whoever obeys Allah and His Messenger will be admitted by Him to gardens [in Paradise] under which rivers flow, abiding eternally therein; and that is the great attainment. And whoever disobeys Allah and His Messenger and transgresses His limits -- He will put him into the Fire to abide eternally therein, and he will have a humiliating punishment.}

Ibn Katheer, may Allah have mercy upon him, said, "There are narrations that encourage learning the obligatory shares of inheritance, the most important of which are these (mentioned in verses 11-12 [in the same Chapter]).

In relation to this, it was reported by Abu Daawood and Ibn Maajah, may Allah have mercy upon them, on the authority of ‘Abdullaah ibn ‘Amr, may Allah be pleased with him, that the Prophet, sallallahu ‘alayhi wa sallam, said: “Three things are essential to learn, and what is beyond them is just favorably optional: a precise (Quranic) Verse, a standing Sunnah or a just prescribed obligatory share (of inheritance).”

The Prophet, sallallahu ‘alayhi wa sallam, ordered that the obligatory shares of inheritance should be distributed among those entitled to them. He said: “Give the prescribed share of inheritance to those who are eligible for it and the remainder [of it] is for the nearest male blood relative.”

The scholars called the science of the obligatory shares (of inheritance) half of the knowledge. Ibn ‘Üyaynah, may Allah have mercy upon him, said, "The science of the obligatory shares (of inheritance) is called half of the knowledge because all the people are in need of it."