Mahasi Sayadaw wrote:The fourth protection for your psychological benefit is to reflect on the phenomenon of ever-approaching death. Buddhist teachings stress that life is uncertain, but death is certain; life is precarious but death is sure. Life has death as its goal. There is birth, disease, suffering, old age, and eventually, death. These are all aspects of the process of existence.
From: Practical Vipassana Meditation Exercises by Mahasi Sayadaw
the Buddha wrote:"For one who has taken birth, great king, there is nothing other [to expect] than aging and death."
From: Samyutta Nikaya 3.3 transl. Bhikkhu Bodhi