A grandmother came to AbuBakr asking him for her share of inheritance. He said: There is nothing prescribed for you in Allah's Book, nor do I know anything for you in the Sunnah of the Prophet of Allah ﷺ Go home till I question the people. He then questioned the people, and al-Mughirah ibn Shu'bah said: I had been present with the Messenger of Allah ﷺ when he gave grandmother a sixth. AbuBakr said: Is there anyone with you? Muhammad ibn Maslamah stood and said the same as al-Mughirah ibn Shu'bah had said. So AbuBakr made it apply to her. Another grandmother came to Umar ibn al-Khattab asking him for her share of inheritance. He said: Nothing has been prescribed for you in Allah's Book. The decision made before you was made for a grandmother other than you. I am not going to add in the shares of inheritance; but it is that sixth. If there are two of you, it is shared between you, but whichever of you is the only one left gets it all.  

مَا لَكِ فِي كِتَابِ اللَّهِ تَعَالَى شَىْءٌ وَمَا عَلِمْتُ لَكِ فِي سُنَّةِ نَبِيِّ اللَّهِ ﷺ شَيْئًا فَارْجِعِي حَتَّى أَسْأَلَ النَّاسَ فَسَأَلَ النَّاسَ فَقَالَ الْمُغِيرَةُ بْنُ شُعْبَةَ حَضَرْتُ رَسُولَ اللَّهِ ﷺ أَعْطَاهَا السُّدُسَ فَقَالَ أَبُو بَكْرٍ هَلْ مَعَكَ غَيْرُكَ فَقَامَ مُحَمَّدُ بْنُ مَسْلَمَةَ فَقَالَ مِثْلَ مَا قَالَ الْمُغِيرَةُ بْنُ شُعْبَةَ فَأَنْفَذَهُ لَهَا أَبُو بَكْرٍ ثُمَّ جَاءَتِ الْجَدَّةُ الأُخْرَى إِلَى عُمَرَ بْنِ الْخَطَّابِ ؓ تَسْأَلُهُ مِيرَاثَهَا فَقَالَ مَا لَكِ فِي كِتَابِ اللَّهِ تَعَالَى شَىْءٌ وَمَا كَانَ الْقَضَاءُ الَّذِي قُضِيَ بِهِ إِلاَّ لِغَيْرِكِ وَمَا أَنَا بِزَائِدٍ فِي الْفَرَائِضِ وَلَكِنْ هُوَ ذَلِكَ السُّدُسُ فَإِنِ اجْتَمَعْتُمَا فِيهِ فَهُوَ بَيْنَكُمَا وَأَيَّتُكُمَا خَلَتْ بِهِ فَهُوَ لَهَا  

The Prophet ﷺ appointed a sixth to a grandmother if no mother is left to inherit before her.  

أَنَّ النَّبِيَّ ﷺ جَعَلَ لِلْجَدَّةِ السُّدُسَ إِذَا لَمْ تَكُنْ دُونَهَا أُمٌّ