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: Sahih Muslim 1627 d
In-book reference: Book 25, Hadith 4
USC-MSA web (English) reference(deprecated numbering scheme): Book 13, Hadith 3990