Musnad Ahmad 8:63

Al-Husayn ibn Ali narrated to us, from Za'idah, from 'Ata' ibn al-Sa'ib, from Abu Abd al-Rahman al-Sulami, who said: Sa'd said regarding the Sunnah of the Messenger of Allah (peace and blessings of Allah be upon him) concerning the third: He came to visit me, and he said to me: "Have you made a will?" I said: "Yes, I have placed all of my wealth for the poor, the needy, and the wayfarer." He said: "Do not do that." I said: "My heirs are wealthy." I said: "Two-thirds?" He said: "No." I said: "Then half?" He said: "No." I said: "The third?" He said: "The third, and the third is much."