We should always remember that death could come to us at any time. The Prophet, peace and blessings of Allah be upon him, advised Abu Ayyub, may Allah be pleased with him:
‘When you stand up to pray, pray a farewell prayer.’ (Recorded in Ahmad)
The ‘farewell prayer’ means the prayer of one who thinks that they will not live to pray again. The person who is praying will no doubt that they are about to die will definitely have khushu.
Sheikh al-Albani, may Allah have mercy on him, recorded in his al-Silsilat al-Sahihah, that the Prophet peace and blessings of Allah be upon him said:
‘Remember death in your prayer, for the man who remembers death during his prayer will surely pray properly, and pray the prayer of a man who does not think that he will pray any other prayer.’ (Reported from al-Suyuti)