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 any Muslim who has any wealth worth bequeathing to spend even two nights without having his bequest written down and kept with him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12588