Story of King Janashruti and cart puller Raikva is found in fourth chapter of Chandogya Upanishad. King Janashruti had attained luminous spiritual power by offering food to people. He built dharmasalas and eateries in all villages and towns in his country to ensure that no one slept without food.
One day the king was resting on the terrace of his palace. Two swans happened to be flying over the palace. One swan warned the other swan not to go near the king and disturb him, as his spiritual power will destroy it.
The other swan retorted by saying that the king is not as great as you think as there is a cart puller named Raikva in whom all virtue and wisdom resided. He is the greatest.
King Janashruti overheard the conversation and was curious to meet cart puller Raikva. After long search, the king was able to locate Raikva. The king made several offers to entice Raikva to make him his disciple. But the king was unsuccessful. The king then threatened the cart puller but it also had no effect on him.
Finally, the king shed his kingly powers and appeared before Raikva as a simple man. Now Raikva relented and taught him many truths.
The teachings are found in the Chandogya Upanishad.