Ikrimah (may Allah have mercy on him) said: Ibn Abbas (may Allah be pleased with them both) sent me to Zayd bin Thabit (may Allah be pleased with him) to ask him about a husband and parents. Zayd (may Allah be pleased with him) said: "For the husband a half, for the mother a third of the remainder, and for the father the rest of the estate." Ibn Abbas (may Allah be pleased with them both) said: "For the mother a third of the entire estate."
In one narration: For the mother a third of the remainder, which is a sixth. Ibn Abbas (may Allah be pleased with them both) sent a message to Zayd (may Allah be pleased with him): "Do you find this in the Book of Allah?" He said: "No, but I dislike giving a mother preference over a father." And Ibn Abbas (may Allah be pleased with them both) used to give the mother a third of the entire estate.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12305