Katha Upanishad, also known as Kathakopanishad, contains the famous conversation between young Nachiketas and Yama, the god of death. Here are few teachings from the Katha Upanishad.
What is the value of wealth or life, as there are impermanent? So long as death is in power we cannot enjoy wealth or life for the fear of death destroys the zest for living. Teach me self knowledge which is beyond the power of death (I.1.27)
Both the good and the pleasant approach a man. The wise man, pondering over them, discriminates. The wise chooses the good in preference to the pleasant. (I.2.2)
He who is filled with selfish desires and attracted by worldly possessions becomes subject to the law of karma which leads him from birth to birth and so he is under the control of Yama.
One who has not desisted from bad conduct, whose senses are not under control, whose mind is not concentrated, whose mind is not free from anxiety (about the result of concentration), cannot attain this Self.
Smaller than the small, greater than the great, the self is set in the heart of every creature. The unstriving man beholds Him, freed from sorrow. Through tranquility of the mind and the senses he sees the greatness of the Self.