Mbh 13.3.1a

"'Yudhishthira said, "If, O prince, Brahmanahood be so difficult of attainment by the three classes (Kshatriyas, Vaisyas and Sudras), how then did the high souled Viswamitra, O king, though a Kshatriya (by birth), attain to the status of a Brahmana? I desire to know this, O sire. Therefore, do thou truly relate this matter to me. That powerful man, O sire, by virtue of his austerities, destroyed in a moment the hundred sons of the high-souled Vasishtha. While under the influence of anger, he created numerous evil spirits and Rakshasas of mighty vigour and resembling the great destroyer Kala himself. The great and learned race of Kusika, numbering hundreds of regenerate sages and belauded by the Brahmanas, was founded in this world of men by him. Sunasepha of austere penances, the son of Richika, having been sought to be slain as an animal in the great sacrifice of Amvarisha, obtained his deliverance through Viswamitra. Harischandra, having pleased the gods at a sacrifice, became a son of the wise Viswamitra. For not having honoured their eldest brother Devarat, whom Viswamitra got as a son from the gods, the other fifty brothers of his were cursed, and all of them became Chandalas. Trisanku, the son of Ikshwaku, through the curse of Vasishtha became a Chandala, and when abandoned by his friends, and remaining suspended with his head downwards in the lower regions, was translated to heaven at the pleasure of Viswamitra. Viswamitra had a large river, by name Kausika, that was frequented by celestial Rishis. This sacred and auspicious stream was frequented by the gods and regenerate Rishis. For disturbing his devotions, the famous celestial nymph Rambha of fine bracelets, was cursed and metamorphosed into a rock. Through fear of Viswamitra the glorious Vasishtha, in olden times, binding himself with creepers, threw himself down into a river and again rose released from his bonds.