Vyasasmriti 2.11

2.11

udvahet kṣattriyāṃ vipro vaiśyāñca kṣattriyo viśām
na tu śūdrāṃ dvijaḥ kaścinnādhamaḥ pūrvavarṇajām

A brahmin may marry a Kṣatriya woman; a Kṣatriya woman may be married by a Vaiśya, [and a Vaiśya woman by a Vaiśya];
no twice-born man may ever marry a Śūdra woman, nor may a man of lower class marry a woman born of a higher class.