Śatapatha Brāhmaṇa ("of a hundred paths") 8.2.3.9

8.2.3.9

yadveva candasyā upadadhāti | prajāpatervisrastātpaśava udakrāmaṃścandāṃsi bhūtvā tāngāyatrī cando bhūtvā vayasāpnottadyadgāyatryāpnodetaddhi canda āśiṣṭhaṃ sā tadbhūtvā prajāpatiretānpaśūnvayasāpnot

And, again, as to why he lays down Khandasyâs. When Pragâpati was relaxed, the cattle, having become metres, went from him. Gâyatrî, having become a metre, overtook them by dint of her vigour; and as to how Gâyatrî overtook them, it is that this is the quickest (shortest) metre. And so Pragâpati, in the form of that (Gâyatrî), by dint of his vigour, overtook those cattle.