Ibn Umar (Allah be pleased with them) reported Allah's Messenger (ﷺ) 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. (ID)
Reference (ID): Sahih Muslim 1627 a (ID)
In-book reference (ID): Book 25, Hadith 1 (ID)
USC-MSA web (English) reference(deprecated numbering scheme): Book 13, Hadith 3987 (ID)