Bhagavad Gita

ದೇಹಿನೋSಸ್ಮಿನ್ ಯಥಾ ದೇಹೇ ಕೌಮಾರಂ ಯೌವನಂ ಜರಾ ।
ತಥಾ ದೇಹಾಂತರಪ್ರಾಪ್ತಿರ್ಧೀರಸ್ತತ್ರ ನ ಮುಹ್ಯತಿ ॥೧೩॥
dehinoSsmin yathā dehe kaumāraṃ yauvanaṃ jarā |
tathā dehāṃtaraprāptirdhīrastatra na muhyati ||13||

Gist of the sloka:
Soul which is present in this body, encounters the body as it changes from birth, youth, age and finally death. One who understands this overcomes the fear of death.
Once the body is destroyed, the soul is not. It continues ahead. Soul which has seen the changes to the body since birth should realize that change is natural and so is death. Soul continues to obtain a new body and starts the bodily journey again.
The fear is due to lack of right knowledge regarding this transmigration of the soul and lack of knowledge about the future body that the soul might get. One who has this knowledge would never fear death for he knows that soul continues to exist.
Attachment [abhimana] causes the fear.