It was narrated from Ibn 'Umar that the Messenger of Allah (ﷺ) said:
"The Muslim man has no right to spend two nights, if he has something for which a will should be made, without having a written will with him." (RU)
English reference (RU): Vol. 3, Book 22, Hadith 2699 (RU)
Arabic reference (RU): Book 22, Hadith 2803 (RU)