It is a duty upon every sane person to be prepared for his departure (from this world), for one does not know when he will be served by his Lord’s commands (to take away his soul) nor does he know how long it is going to be before he will be summoned (for the Day of Judgment).

Taken from Jami’ al-U’lum wal-Hikam [With the authentication of Salim al-Hilali.
Hadeeth #40:

Ibn Umar (Radhiallahu ‘Anhum) said: «Allaah’s Messenger (Sallallahu ‘Alaihi Wa Sallam) took me by my shoulder and said: “Be in this life as if you were a stranger or a traveller on a path.”