2-Satyakaam
His story comes in Chhaandogya Upanishad

There lived a woman named Jaabaal. She had a son named Satyakaam. He wished to study as his other mates studied, but he could not find any. So he left home in search of a Guru. He went to the Aashram of sage Gautam and requested him to make him his disciple. Gautam Rishi asked him - "Before I make you my disciple, I need to know about your family, who is your father, what is your Gotra etc etc." This puzzled the young boy Satyakaam, as he had no idea about his family except his mother, but as he had a strong desire to be his disciple he would not lie. He told the sage that he would ask his mother about it. So he went to his mother.

His mother told him, "Go to the sage and tell him - "I am the son of Jaabaal and my name is Satyakaam Jaabaali. My mother had served many people so she does not know the name of my father." Satyakaam went to back to Rishi Gautam and told him what his mother told him to tell him. The sage was so pleased with his love for truth that he immediately accepted him as his disciple. He could not believe that a mother could be so truthful to his son; and that son also could be so truthful to repeat it to strangers without feeling any guilt or humiliation. He took the decision with utter understanding so he said - "You are accepted, O Satyakaam, because at the age of 9 you are so truthful."

One day Gautam told him that he should take the herd of 400 weak cows of the Aashram and return only when it had multiplied to 1000. Only after that Gautam would impart him knowledge. Without uttering a single word Satyakaam left with the cows to a forest. He took them to the forest and looked after them with loving care. he  built an Aashram there and took a great care of Guru Jee's cows. He also followwed all the rules of Brahmchaaree. Years went by and one day he found that his cows have now grown to 1,000. Every cow was strong and healthy. Now it was time for Satyakaam to return to Gautam's Aashram. So he drove his cows and set on his journey to the Aashram.

Satyakaam Obtains Gyaan
All the gods and deities were very happy with Satyakaam's obedience and dedication to his Guru. So he was coming to Guru Jee, he halted at one place for night. He lighted a warm fire and sat near it. After a while Agni Dev came out of that fire and taught him Ved and Shaastra. It was only 1/4th of the knowledge. Agni Dev said - "You will obtain further knowledge by somebody else." and disappeared.

Next day he again halted for night. This time he stayed near a lake. After a while a swan appeared on the lake and he blessed and enlightened him. This was also 1/4th of the Gyaan, he told him that he would receive another 1/4th Gyaan by somebody else. Next day he again halted for night. There he was enlightened by a bull and a water fowl. The fourth day he came to his Guru Jee.

Now Satyakaam was enlightened. When he reached the Aashram he had a Tej around his face. Gautam saw the glow of enlightenment on his face. He knew everything through his Yog, still he wanted to hear from Satyakaam's mouth. He asked Satyakaam - "You have a great Tej on your face, how is that? Who taught you?" Satyakaam bowed to his Guru and told him all that happened during his journey to Aashram. Guru Jee was very happy to hear that Satyakaam had looked after the cows very well and got the Gyaan without teaching. Satyakaam now requested him to gteach. Guru Jee said - "O Satyakaam, Now there is nothing left to teach you, you have already known everything. In fact you know more than me." Then he imparted Satyakaam the last part of his education - Brahmavidyaa.

[Thus by speaking truth Satyakaam became the disciple of a great Guru and got Brahm Gyaan without studying anything.]