Sahih Muslim
The Book of Bequests (Kitab Al-Wasiyya)
Book Number: 013 / Hadith Number: 3987
Ibn Umar (Allah be pleased with them) reported Allah's Messenger (may peace be upon him) as saying: It is the duty of a Muslim who has something which is to be given as a bequest not to have it for two nights without having his will written down regarding it.