Sahih Muslim Book 13, Hadith Number 3990.

Chapter : One should make it a point to write the will.

Salim reported on the authority of his father (‘Abdullah b. Umar) that he (his father) had heard Allah’s Messenger (may peace be upon him) as saying: It is not proper for a Muslim who has got something to bequeath to spend even three nights without having his will written down with him regarding it. ‘Abdullah b. ‘Umar (Allah be pleased with them) said: Ever since I heard Allah’s Messenger (may peace be upon him) say this I have not spent a night without having my will (written) along with me. This hadith has been narrated on the authority of Zuhri with the same chain of transmitters.