Remember death in your prayer, for the man who remembers death during his prayer is bound to pray properly, and pray the prayer of a man who does not think that he will pray any other prayer.
 

[al-Silsilat al-Saheehah by al-Albaani, 1421. It was reported from al-Suyooti that al-Haafiz ibn Hajar classed this hadeeth as hasan]