Sahih Muslim Book 013, Hadith Number 3998.
Chapter : Will can be made only for one-third.
Humaid b. Abd al-Rahmin al-Himayri reported on the authority of the three of the sons of Sa'd: They said: Sa'd fell ill in Mecca. Allah's Messenger (may peace be upon him) visited him to inquire after his health. The rest of the hadith is the same.