It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "It is not right for a Muslim who has something worth bequeathing to spend three nights without having written his bequest." Ibn Umar (may Allah be pleased with them both) said: Since I heard this from the Messenger of Allah (peace and blessings be upon him), not a single night has passed over me without my written bequest being with me.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12590