Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.4.19

4.2.4.19

devā ha vai yajñaṃ tanvānāḥ | te 'surarakṣasebhya āsaṅgādbibhayāṃ cakrustāndakṣiṇato 'surarakṣasānyāsejusteṣāmetāndakṣiṇāngrahānujjaghnurapyetaddakṣiṇaṃ havirdhānamujjaghnurathaitameva na śekuruddhantuṃ taduttarameva havirdhānaṃ dakṣiṇaṃ havirdhānamadṛṃhattadyadetaṃ na śekuruddhantuṃ tasmāddhruvo nāma

Now, once on a time, the gods, while performing sacrifice, were afraid of an attack from the Asura-Rakshas. The Asura-Rakshas assailed them from the south, and overturned those southern cups of Soma,--even that southern Soma-cart they overturned; but that other (cart) they could not overturn: the northern cart then kept the southern cart steady. And because they could not overturn that (northern cup) therefore it is called Dhruva (firm).