Mbh 9.40.2b

' Thus addressed, Gadhi replied unto his subjects, saying, 'My son will become the protector of the wide universe!' Having said these words, and placed Vishvamitra (on the throne), Gadhi, O king, went to heaven, and Vishvamitra became king. He could not, however, protect the earth with even his best exertions. The king then heard of the existence of a great fear of Rakshasas (in his kingdom). With his four kinds of forces, he went out of his capital. Having proceeded far on his way, he reached the asylum of Vasishtha. His troops, O king, caused much mischief there. The adorable Brahmana Vasishtha, when he came to his asylum, saw the extensive woods in course of destruction. That best of Rishis, Vasishtha, O king, became angry, O monarch, with Vishvamitra. He commanded his own (homa) cow, saying, 'Create a number of terrible Savaras!' Thus addressed, the cow created a swarm of men of frightful visages. These encountered the army of Vishvamitra and began to cause a great carnage everywhere. Seeing this, his troops fled away. Vishvamitra, the son of Gadhi, however, regarding ascetic austerities highly efficacious, set his heart upon them. In this foremost of tirthas of the Sarasvati, O king, he began to emaciate his own body by means of vows and fasts with fixed resolve. He made water and air and (the fallen) leaves of trees his food. He slept on the bare ground, and observed other vows (enjoined for ascetics). The gods made repeated attempts for impeding him in the observance of his vows. His heart, however, never swerved from the vows (he had proposed to himself). Then, having practised diverse kinds of austerities with great devotion, the son of Gadhi became like the Sun himself in effulgence. The boon-giving Grandsire, of great energy, resolved to grant Vishvamitra, when he had become endued with ascetic merit, the boon the latter desired. The boon that Vishvamitra solicited was that he should be permitted to become a Brahmana.