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." (AR)
English reference (AR): Vol. 3, Book 22, Hadith 2699 (AR)
Arabic reference (AR): Book 22, Hadith 2803 (AR)