Anushasana Parva 44.12

Of the three wives of a Brahmana. She taken from his own caste should be considered as the foremost. Likewise, of the two wives permitted to the Kshatriya, she taken from his own caste should be considered as superior. Some say that persons belonging to the three higher castes may take, only for purposes of enjoyment, wives from the lowest or the Shudra caste. Others, however, forbid the practice.