Manava Dharmashastra 3.13

3.13

śūdraiva bhāryā śūdrasya sā ca svā ca viśaḥ smṛte
te ca svā caiva rājñaś ca tāś ca svā cāgrajanmanaḥ //

It is declared that a Sudra woman alone (can be) the wife of a Sudra, she and one of his own caste (the wives) of a Vaisya, those two and one of his own caste (the wives) of a Kshatriya, those three and one of his own caste (the wives) of a Brahmana.