Shatapatha Brahmana 8.1.4.7

8.1.4.7

athātaḥ samañcanaprasāraṇasyaiva | saṃcitaṃ haike samañcanaprasāraṇenetyabhimṛśanti paśureṣa yadagniryadā vai paśuraṅgāni saṃ cāñcati pra ca sārayatyatha sa tairvīryaṃ karoti

Now, then, as to the contraction and expansion (of the body). Now some cause the built (altar) in this way to be possessed of (the power of) contraction and expansion: that Agni indeed is an animal; and when an animal contracts and expands its limbs, it develops strength by them.