Musnad Ahmad 27:1062

Yahya ibn Sa'id al-Umawi narrated to us, 'Ubayd Allah narrated to us, from Nafi', from Ibn 'Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "It is not the right of a Muslim man who has something to bequeath to spend two nights except that his will is written with him."