Mahabharata 3.141.1
युधिष्ठिर उवाच · yudhiṣṭhira uvāca
अन्तर्हितानि भूतानि रक्षांसि बलवन्ति चअग्निना तपसा चैव शक्यं गन्तुं वृकोदर
antarhitāni bhūtāni rakṣāṃsi balavanti caagninā tapasā caiva śakyaṃ gantuṃ vṛkodara
Our plain-language meaning of this verse is waiting for scholar review.