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

Ibn Umar (Allah be pleased with them) reported Allah's Messenger (PBUH) as saying: It is the duty of a Muslim who has something which is to be given as a bequest not to have it for two nights without having his will written down regarding it. Translation Not Available

Book reference: Sahih Muslim Book 25 Hadith 3987
Web reference: Sahih Muslim Book 13 Hadith 1627 a