“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 he will pray any other prayer.”1
“When you stand up to pray, pray as a farewell prayer.”2
Meaning that the prayer of one who thinks that he will not pray another prayer i.e., the person praying will no doubt die, and there is some prayer that will be his last prayer, so let him have khushoo (concentration) in the prayer that he is doing, for he does not know whether or not his will be his last prayer.
1 Silsilat alhadeeth as-Saheehah by Shaykh al-Albani, hadith number 1421. It is reported from as-Suyooti that al-Hafidh ibn Hajarclassed this hadith as hasan.
2 Ahmad, 5/412; Saheeh al-Jami, hadith number 742
From the book “33 ways of developing Khushoo” By Muhammd Salih Al-Munajjid.