Sahih Muslim Book 013, Hadith Number 3994.
Chapter : Will can be made only for one-third.
Mus'ab b. Sa'd reported on the authority of his father. I was ailing. I sent message to Allah's Apostle (may peace be upon him) saying: Permit me to give away my property as I like. He refused. I (again) said: (Permit me) to give away half. He (again refused). I (again said): Then one-third. He (the Holy Prophet) observed silence after (I had asked permission to give away) one-third. He (the narrator) said: It was then that endowment of one-third became permissible.