Maliks Muwatta Book 27, Hadith Number 5.

Yahya related to me from Malik from Yahya ibn Said that al-Qasim ibn Muhammad said, “Two grandmothers came to Abu Bakr asSiddiq, and he wanted to give the sixth to the one who was from the mother’s side, and a man of the Ansar said, ‘What? Are you omitting the one from whom he would inherit if she died while he was alive?’ Abu Bakr divided the sixth between them.”