Sunnah.com reference : Book 7, Hadith 222English translation : Book 7, Hadith 965Arabic reference : Book 7, Hadith 958
Narrated Ibn 'Umar (RA):
Allah's Messenger (ﷺ) said: "It is not permissible for a Muslim person, who has something he wants to give as a bequest, to have it for two nights without having his will regarding it written and kept ready with him."

[Agreed upon].
عَنْ اِبْنِ عُمَرَ ‏- رَضِيَ اَللَّهُ عَنْهُمَا‏- ; أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ : { مَا حَقُّ اِمْرِئٍ مُسْلِمٍ لَهُ شَيْءٌ يُرِيدُ أَنْ يُوصِيَ فِيهِ يَبِيتُ لَيْلَتَيْنِ إِلَّا وَوَصِيَّتُهُ مَكْتُوبَةٌ عِنْدَهُ }  مُتَّفَقٌ عَلَيْهِ  1‏ .‏‏1 ‏- صحيح .‏ رواه البخاري ( 2738 )‏ ، ومسلم ( 1627 )‏.‏