Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.3.17

1.7.3.17

te vai triṣṭubhau bhavataḥ | vāstu vā etadyajñasya yatsviṣṭakṛdavīryaṃ vai vāstvindriyaṃ vīryaṃ triṣṭubindriyamevaitadvīryaṃ vāstau sviṣṭakṛti dadhāti tasmāttriṣṭubhau bhavataḥ

They are both trishtubh verses; for the svishtakrit is, as it were, the residue (or site, vâstu) of the sacrifice, and the residue (or, a vacant site) is without energy. Now the trishtubh means manly power, energy: hence he thereby imparts manly power, energy to that residue, the svishtakrit. This is why they are both trishtubh verses.