Kena (Talavakāra) Upaniṣad 3.6

3.6

tasmai tṛṇaṃ nidadhāv
etad daheti
tad upapreyāya sarvajavena
tan na śaśāka dagdhuṃ
sa tata eva nivavṛte
naitad aśakaṃ vijñātuṃ
yad etad yakṣam iti

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