Saurapurana 17.54

17.54

tamācāraṃ niveveta sādhūnvakṣyāmi sāṃpratam
gaṅgāyamunayormadhye madhyadeśaḥ prakīrtitāḥ

That conduct he should practice; now I shall describe the good. / The region between the Gaṅgā and the Yamunā is celebrated as the Middle Country;