One-third is mentioned in the Quran in these verses:

[4:11] God decrees a will for the benefit of your children; the male gets twice the share of the female. If the inheritors are only women, more than two, they get two-thirds of what is bequeathed. If only one daughter is left, she gets one-half. The parents of the deceased get one-sixth of the inheritance each, if the deceased has left any children. If he left no children, and his parents are the only inheritors, the mother gets one-third. If he has siblings, then the mother gets one-sixth. All this, after fulfilling any will the deceased has left, and after paying off all debts. When it comes to your parents and your children, you do not know which of them is really the best to you and the most beneficial. This is God’s law. God is Omniscient, Most Wise.

[4:12] You get half of what your wives leave behind, if they had no children. If they had children, you get one-fourth of what they leave. All this, after fulfilling any will they had left, and after paying off all debts. They get one-fourth of what you leave behind, if you had no children. If you had children, they get one-eighth of what you bequeath. All this, after fulfilling any will you had left, and after paying off all debts. If the deceased man or woman was a loner, and leaves two siblings, male or female, each of them gets one-sixth of the inheritance. If there are more siblings, then they equally share one-third of the inheritance. All this, after fulfilling any will, and after paying off all debts, so that no one is hurt. This is a will decreed by God. God is Omniscient, Clement.

[73:20] Your Lord knows that you meditate during two-thirds of the night, or half of it, or one-third of it, and so do some of those who believed with you. God has designed the night and the day, and He knows that you cannot always do this. He has pardoned you. Instead, you shall read what you can of the Quran. He knows that some of you may be ill, others may be traveling in pursuit of God’s provisions, and others may be striving in the cause of God. You shall read what you can of it, and observe the contact prayers (Salat), give the obligatory charity (Zakat), and lend God a loan of righteousness. Whatever good you send ahead on behalf of your souls, you will find it at God far better and generously rewarded. And implore God for forgiveness. God is Forgiver, Most Merciful.

To learn more, read the Quran in English.

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