Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 6.2.2

6.2.2

iti netihaivovāca vettho yatithyāmāhutyām̐ hutāyāmāpaḥ puruṣavāco bhūtvā samutthāya vadantī3 iti neti haivovāca vettho devayānasya vā yathaḥpratipadaṃ pitṛyāṇasya vā yatkṛtvā devayānaṃ vā panthānaṃ pratipadyante pitṛyāṇaṃ vāpi hi na ṣervacaḥ śrutam | dve sṛtī aśṛṇavaṃ pitṛṇāmahaṃ devānāmuta martyānām | tābhyāmidaṃ viśvamejatsameti yadantarā pitaraṃ mātaraṃ ceti nāhamata ekaṃcana vedeti hovāca

The king said: 'Do you know how men, when they depart from here, separate from each other?' 'No,' he replied.
'Do you know how they come back to this world?' 'No,' he replied.
'Do you know how that world does never become full with the many who again and again depart thither?' 'No,' he replied.
'Do you know at the offering of which libation the waters become endowed with a human voice and rise and speak?' 'No,' he replied.
'Do you know the access to the path leading to the Devas and to the path leading to the Fathers, i.e. by what deeds men gain access to the path leading to the Devas or to that leading to the Fathers? For we have heard even the saying of a Rishi: "I heard of two paths for men, one leading to the Fathers, the other leading to the Devas. On those paths all that lives moves on, whatever there is between father (sky) and mother (earth)."'
Svetaketu said: 'I do not know even one of all these questions.'