Mahabharata 1.98.5

1.98.5

pāṇigrāhasya tanaya iti vedeṣu niścitam |
dharmaṃ manasi saṃsthāpya brāhmaṇāṃstāḥ samabhyayuḥ |
loke'pyācarito dṛṣṭaḥ kṣatriyāṇāṃ punarbhavaḥ || 5 ||

"And when the earth was thus deprived of Kshatriyas by the great Rishi, the Kshatriya ladies all over the land had offspring raised by Brahmanas skilled in the Vedas. It has been said in the Vedas that the sons so raised belongeth to him that had married the mother. And the Kshatriya ladies went in unto the Brahamanas not lustfully but from motives of virtue. Indeed, it was thus that the Kshatriya race was revived.