It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "It is not proper for a Muslim who has something to bequeath to spend two nights without having his will written with him."
Hadith Referenceالسنن المأثورة / باب ما جاء في فدية الأذى / 512
Hadith Takhrijصحیح بخاری، الوصايا، باب الوصايا، رقم: 2738