Śatapatha Brāhmaṇa ("of a hundred paths") 8.2.3.14

8.2.3.14

tāni vā etāni | catvāri vayāṃsi catvāri candāṃsi tadaṣṭāvaṣṭākṣarā gāyatryeṣā vai sā gāyatrī yā tadbhūtvā prajāpatiretānpaśūnvayasāpnottasmājjīrṇam paśuṃ vayasāpta ityācakṣate tasmādu sarvāsveva vayo vaya ityamuvartate 'tha ye 'smātte paśava udakrāmannete te pañcadaśottare vajro vai paśavo vajraḥ pañcadaśastasmādyasya paśavo bhavantyapaiva sa pāpmānaṃ hate vajro haiva tasya pāpmānamapahanti tasmādyāṃ kāṃ ca diśam paśumāneti vajravihitāṃ haiva tāmanveti

These then are four kinds of vigour, and four metres; this (makes) eight,--the Gâyatrî consists of eight syllables: this, assuredly, is that same Gâyatrî in the form of which Pragâpati then, by his vigour, overtook those cattle; whence they say of worn-out cattle that they are overtaken by vigour (or, age), and hence (the word) 'vigour' recurs with all (these bricks). And those cattle which went away from him (Pragâpati) are these fifteen other (formulas): the cattle are a thunderbolt, and the thunderbolt is fifteenfold: whence he who possesses cattle, drives off the evildoer, for the thunderbolt drives off the evildoer for him. And in whatever direction, therefore, the possessor of cattle goes, that he finds torn up by the thunderbolt.