Sahih Muslim Book - 25, Hadith - 4204

حَدَّثَنَا أَبُو خَيْثَمَةَ زُهَيْرُ بْنُ حَرْبٍ وَمُحَمَّدُ بْنُ الْمُثَنَّى الْعَنَزِىُّ - وَاللَّفْظُ لاِبْنِ الْمُثَنَّى - قَالاَ حَدَّثَنَا يَحْيَى - وَهُوَ ابْنُ سَعِيدٍ الْقَطَّانُ - عَنْ عُبَيْدِ اللَّهِ أَخْبَرَنِى نَافِعٌ عَنِ ابْنِ عُمَرَ أَنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- قَالَ « مَا حَقُّ امْرِئٍ مُسْلِمٍ لَهُ شَىْءٌ يُرِيدُ أَنْ يُوصِىَ فِيهِ يَبِيتُ لَيْلَتَيْنِ إِلاَّ وَوَصِيَّتُهُ مَكْتُوبَةٌ عِنْدَهُ ».

It was narrated from Ibn 'Umar that the Messenger of Allah (s.a.w) said: "It is not right for a Muslim man who has anything that he wants to bequeath, to stay for more than two nights without having his will written with him."