Sahih Muslim Book 013, Hadith Number 3996.
Chapter : Will can be made only for one-third.
Ibn Sa'd reported his father as saying: Allah's Apostle (may peace be upon him) visited me during my illness. I said: I am willing away the whole of my property. He said: No. I said: Then half? He said: No. I said: Should I will away one-third? He said: Yes, and even one-third is enough.