Yazid al-Rishk (may Allah have mercy on him) said: I asked Sa'id bin al-Musayyab (may Allah have mercy on him) about a man who died and left a wife and parents. Sa'id (may Allah have mercy on him) said: "Zayd (may Allah be pleased with him) divided it into four shares: one share for the wife, a third of the remainder for the mother, and the rest of the estate for the father."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12309