Musnad Ahmad 27:1640

Kuthayyir ibn Hisham narrated to us, Ja'far ibn Burqan narrated to us, al-Zuhri narrated to us, from Salim, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is not the right of a Muslim man who has property to bequeath, that he spends three nights except that his will is written with him." 'Abdullah said: I have not spent a single night since I heard it except that my will is written with me.