It is reported from Sayyiduna Umar and Sayyiduna Abdullah (may Allah be pleased with them both) concerning a woman (wife) and parents: for the mother a third of the remainder. And Sayyiduna Ali (may Allah be pleased with him) said: For the mother is a third of the entire estate.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12304