Kaṭha Upaniṣad 1.20

1.20

yeyaṃ prete vicikitsā manuṣye astīty eke nāyam astīti caike /
etad vidyām anuśiṣṭas tvayāhaṃ varāṇām eṣa varas tṛtīyaḥ //

[Naciketas:] 20. This doubt that there is in regard to a man deceased: ‘He exists,’ say some; ‘He exists not,’ say others— This would I know, instructed by thee! Of the boons this is boon the third.