It was narrated that Abu Abdur-Rahman as-Sulami said: Sa"d said:

It was concerning me that the Messenger of Allah (ﷺ) established the principle of (bequeathing) one-third. He came to visit one when I was sick and said to me: “Did you make a bequest?" I said: Yes, I bequeathed all my wealth to the poor, needy and wayfarers. He said: “Do not do that.“ I said: My heirs are independent of means, [Can I bequeath] two thirds? He said: “No." I said: Half? He said: “No." I said: One third? He said: "One third, and one third is a lot."