Mishkat al-Masabih
Buraida said that the Prophet (ﷺ) appointed a sixth to a grandmother if no mother is left to inherit before her. Abu Dawud transmitted it.
وَعَنْ بُرَيْدَةَ: أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ جَعَلَ لِلْجَدَّةِ السُّدُسَ إِذَا لَمْ تَكُنْ دونهَا أم. رَوَاهُ أَبُو دَاوُد
Reference