Musannaf Ibn Abi Shayba 26:19

Narrated to us Abd al-Salam ibn Harb, from Ayyub, from Abu Qilabah, from Abu al-Muhallab, that Uthman was asked about it, and he said: "For the wife is one-fourth, and for the mother is one-third of what remains, and the rest of that is for the father."