(30) The Book of Wills
كتاب الوصايا
(1) Chapter: It Is Disliked To Delay Making A Will
(1) باب الْكَرَاهِيَةِ فِي تَأْخِيرِ الْوَصِيَّةِ
Sunan an-Nasa'i 3615 
It was narrated that Ibn 'Umar said:
"The Messenger of Allah said: 'It is not befitting for a Muslim who has anything concerning which a will should be made, to abide for two nights without having a written will with him.'"
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا الْفُضَيْلُ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ مَا حَقُّ امْرِئٍ مُسْلِمٍ لَهُ شَىْءٌ يُوصَى فِيهِ أَنْ يَبِيتَ لَيْلَتَيْنِ إِلاَّ وَوَصِيَّتُهُ مَكْتُوبَةٌ عِنْدَهُ ‏"‏‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 3615
In-book reference : Book 30, Hadith 5
English translation : Vol. 4, Book 30, Hadith 3645