Musannaf Ibn Abi Shayba 26:22

Abu Mu'awiyah narrated to us, from al-A'mash, from Ibrahim, who said: Abdullah was approached regarding a woman and two parents. He said: "Umar, whenever he took a path, we would take it and find it easy. He was approached regarding a woman and two parents, so he made it [the inheritance] out of four. He gave the woman a quarter, and the mother a third of what remained, and he gave the father the rest of that."