The Law Code of Yājñavalkya 1.3.42

1.3.42

brāhmaṇasya catasro bhāryāḥ kṣatriyasya tisro vaiśyasya dve iti (MDh 3.13) dvijātīnāṃ śūdrāvedanam iti, na etad yājñavalkyasya matam | yasmād ayaṃ dvijātis tatra svayaṃ jāyate, taj jāyā jāyā bhavati yad asyāṃ jāyate punaḥ (AitB 7.13) iti śruteḥ | atra ca tatrāyaṃ jāyate svayam iti hetuṃ vadatā naityakaputrotpādanāya kāmyaputrotpādanāya vā pravṛttasya śūdrāpariṇayananiṣedhaṃ kurvatā naityakaputrotpādanānukalpe kāmye ca putropādane brāhmaṇasya kṣatriyāvaiśye kṣatriyasya ca vaiśyā bhāryānujñātā bhavati || 1.56 ||

The view that a Brahmin may take four wives, a Kshatriya three, and a Vaishya two—namely, the marrying of a Shudra woman by twice-born men—is not the opinion of Yājñavalkya. For this twice-born man is born again in her, as stated in the Vedic revelation: 'That wife is called a wife (jāyā) because one is born (jāyate) again in her.' And here, by stating the reason 'in her he himself is born', and thereby prohibiting the marrying of a Shudra woman by one who acts to produce an obligatory son or a son born out of desire, a Kshatriya and a Vaishya woman are permitted as wives to a Brahmin, and a Vaishya woman to a Kshatriya, in the secondary alternative for producing an obligatory son and in the production of a son born out of desire. || 1.56