It was narrated from Ibn 'Umar that the Prophet (ﷺ) said:
"The Muslim man has no right to spend two nights, if he has something for which will should be made, without having a written will with him." (IT)
English reference (IT): Vol. 3, Book 22, Hadith 2702 (IT)
Arabic reference (IT): Book 22, Hadith 2806 (IT)