Sunan an-Nasa'i, Hadith 3616 ⓘ
It was narrated from Ibn 'Umar that 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 مُحَمَّدُ بْنُ سَلَمَةَ، قَالَ حَدَّثَنَا ابْنُ الْقَاسِمِ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " مَا حَقُّ امْرِئٍ مُسْلِمٍ لَهُ شَىْءٌ يُوصَى فِيهِ يَبِيتُ لَيْلَتَيْنِ إِلاَّ وَوَصِيَّتُهُ مَكْتُوبَةٌ عِنْدَهُ " .
also referenced under: Book 30, Hadith 6 (In-Book) & Volume 4, Book 30, Hadith 3646 (DarusSalam)