Musnad Ahmad 27:22

Mu'tamir narrated to us, I heard Burayd, from al-Zuhri, from Salim, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "No one should spend three nights except that his will is written with him." He (Ibn Umar) said: So I have not spent a single night since then, except that my will is placed with me.