Majma al-Zawaid — Hadith 7079

Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) narrates this saying of the Prophet (peace and blessings be upon him): "It is not the right of any Muslim man to spend two black nights while he has something about which he could make a will, except that his will should be with him in written form."
Abu Ya'la recorded it in al-Kabir; its chain contains Abdullah al-Umari, in whom there is weakness, though he has been declared trustworthy; the rest of its narrators are the narrators of the Sahih.