KeU 3.6
तस्मै तृणं निदधावेतद्दहेति । तदुपप्रेयाय सर्वजवेन तन्न शशाक दग्धुं स तत एव निववृते नैतदशकं विज्ञातुं यदेतद्यक्षमिति
Brahman placed a straw before him and said: "Burn this." He (Agni) rushed towards it with all speed, but was not able to burn it. So he returned from there and said (to the Devas): "I was not able to find out what this great mystery is."
Translated by Swami Paramananda
Brahman put a straw before him, saying: 'Burn this.' He went towards it with all his might, but he could not burn it. Then he returned thence and said: 'I could not find out what sprite this is.'
Translated by F. Max Müller