Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.3.5

2.1.3.5

brahmaiva vasantaḥ | kṣatraṃ grīṣmo viḍeva varṣāstasmādbrāhmaṇo vasanta ādadhīta brahma hi vasantastasmātkṣatriyo grīṣma ādadhīta kṣatraṃ hi grīṣmastasmādvaiśyo varṣāsvādadhīta viḍḍhi varṣāḥ

The spring is the priesthood, the summer the nobility, and the rainy season the common people (vis): a Brâhman therefore should set up his fires in spring, since the spring is the priesthood; and a Kshatriya should set them up in summer, since the summer is the nobility; and a Vaisya should set them up in the rainy season, since the rainy season is the common people.