Mishkat al-Masabih 12 Inheritance and Wills كتاب الفرائض والوصايا 29

عَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَا حَقُّ امْرِئٍ مُسْلِمٍ لَهُ شَيْءٌ يُوصَى فِيهِ يَبِيتُ لَيْلَتَيْنِ إِلَّا وَوَصِيَّة مَكْتُوبَة عِنْده»

Wills. Ibn ‘Umar reported God’s Messenger as saying, “It is the duty of a Muslim man who has something which is to be given as a bequest not to have it for two nights without having his will written regarding it.” (Bukhari and Muslim.)