The Law Code of Yājñavalkya 1.2.3

1.2.3

brāhmaṇakṣatriyavaiśyaśūdrāś catvāro varṇā vakṣyamāṇalakṣaṇās, teṣām ādyās trayo brāhmaṇakṣatriyavaiśyā dvijāḥ | dvir jāyanta iti dvijāḥ | teṣāṃ dvijānāṃ vai eva na śūdrasya | etena śūdrasyāmantrakāḥ kriyā ity uktaṃ bhavati |

Brahmins, Kshatriyas, Vaishyas, and Shudras are the four classes whose characteristics will be explained. Among them, the first three—Brahmins, Kshatriyas, and Vaishyas—are twice-born. They are called twice-born because they are born twice. Their rites—only those of the twice-born, not of the Shudra; by this it is stated that the Shudra's rites are without mantras.