Katha Upanishad:
In the Katha Upanishad, the sage dramatically explains Brahmajnana through the dialogue between Nachiketa and Yamaraja. Nachiketa's main question is about life after death. The same thing is playing in our mind that there is really life after death or not? We are trying to find a solution for this.
In order to make it clear that there is life after death, this Upanishad describes our constitution and the all-powerful arrangement of God, in which the fruits of our deeds of this life are preserved and we get them in the form of nature and faith in the new birth, and if we do our deeds without attachment, the fruits of them. It is explained that we can get it in this birth or even in a new birth