Kena (Talavakāra) Upaniṣad 3.10

3.10

tasmai tṛṇaṃ nidadhāv
etad ādatsveti
tad upapreyāya sarvajavena
tan na śaśākādatum
sa tata eva nivavṛte
naitad aśakaṃ vijñātuṃ
yad etad yakṣam iti

23 (10). It put down a straw before him. ‘Carry that off!’ He went at it with all speed. He was not able to carry it off. Thereupon indeed he returned, saying: ‘I have not been able to find out this—what this wonderful being is.’