On one of his exploits, he and his soldiers took rest in the ashram of Rishi Vasistha. There, his whole army was well fed and taken care of. This caused a doubt in the king's mind as to how it was possible for this simple ashram to take care of all the arrangements to feed an entire army. He expressed his surprise to the sage. Vasistha replied,

"O king, this feast that you have partaken with your kinsmen, has been provided by my calf Nandini (sometimes referred as Sabala), who was gifted to me by Indra. You must know that she is the daughter of Indra's cow Kamadhenu. She provides me with everything I need."

Koushika was filled with wonder when he heard this. He began to think that possessing this cow would mean a lot to him; after all, the sage did not have to provide food and sustenance for a large army everyday. He expressed a desire to the sage for obtaining Nandini from him.

Vasishta was polite, but steadfast in his refusal. He would not be tempted by the offer of untold wealth that was made by Koushika, for after all who can set a price on a cow, which can readily yield all the riches in the world. The king grew exceedingly angry. He insulted the Brahmarishi with harsh words, and ordered his soldiers to seize the cow, and drive it to his kingdom. Nandini was the daughter of Kamdhenu and hence she forcefully protested against the soldiers. Using her powers, she saved the life of Vasishta.

This incident made a deep impression on the King. He realized that the power obtained by penances was far greater than mere physical might. He renounced his kingdom and began his quest to become a greater rishi than Vasistha. He took on the name Vishvamitra. It is very interesting to see all the challenges that Viswamitra faced in his life to become a Brahmarishi, before eventually giving up the greed to possess the cow. After many trials and undergoing many austerities, Vishvamitra at last obtained the title of Brahmarishi from Vasistha himself.

During this time he had a daughter named Shakuntala (who appears in the Mahabharata) with Menaka, an apsara in the court of Indra. Son of Shakuntala became a great emperor. He came to be known as Emperor Bharata, in whose name the land of India got its name Bharatha.