From Sahih Muslim (Hadith 4207):
Salim reported on the authority of his father ('Abdullah b. Umar) that he (his father) had heard Allah's Messenger (ﷺ) as saying:It is not proper for a Muslim who has got something to bequeathe 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 (ﷺ) say this I have not spent a night without having my will (written) along with me
Reference: Muslim 4207