There is a story from Dwapar yuga about sage named Dhoumya. It is about how he passed on the knowledge (Atma dnyan) to his disciples after testing their devotion. Being doshadnya i.e. knower of all visible and subtle faults of his disciples, he made his disciples serve him in various ways, not for his own benefit but to purify their hearts and correct their faults so that they become fit to receive the highest knowledge. Real Gurus are embodiments of pure love. Whatever they do for their disciples, be it giving them punishments or giving them hard work to test their devotion, they do it out of love with pure motive of taking them to the highest goal of life and that is attaining atma dnyana (knowledge of Self).
Dhoumya had three disciples - Aruni, Ved and Upamanyu.
To Aruni he commanded to water the fields. Aruni went for his task. He tried all his efforts to direct the water stream to the fields but water had such a force that he could not stop and direct it. Finally he himself lay down in that cold water and succeeded in stopping and directing water to the fields. When Aruni did not return home in time, Dhoumya went out in search of Aruni. When he reached the spot, Dhoumya understood the situation. Pleased by devotion of Aruni, he offered him blessings to attain highest knowledge and all good fortune.
To Ved he commanded to protect fields. Gave him one cart as well, to collect and bring the grains back home. Ved went for his task. Protected fields. And then filled cart with grains and started back to home. On his way back, one of the wheels of cart got stuck in mud. Ved had to put very mighty efforts to get cart out of mud. He replaced one bullock with himself and then tied rope around his own neck and pulled cart out of mud. Knowing it took long time for Ved to return home Dhoumya went out in search and found Ved bringing back cart to home with mighty efforts. Pleased by devotion of Ved, he offered him blessings to attain highest knowledge and all good fortune.
To Upamanyu, he commanded to tend the cattle. Upamanyu started his daily task. Upamanyu had a very large appetite. He used to eat a lot of food. Initially Dhoumya used to send him lunch. But knowing that eating more than required food can badly affect Upamanyu's intelligence he stopped it. To satisfy his own hunger, Upamanyu started begging alms. When Dhoumya came to know this, he commanded Upamanyu to bring alms back to home. Then Upamanyu started begging twice to give one alm back home and one for himself. Dhoumya commanded him to bring both alms back to home. Then Upamanyu started drinking milk left in udder of cows after calves have drank their milk. Dhoumya, upon knowing this, warned Upamanyu of dire consequences, such as becoming as stupid as those creatures, if he continued doing so. Upamanyu thought over and realized that latex of plant is not left over by any animal and hence a good substitute for milk and he started drinking that. But while doing that some drops of poisonous oozing went into his eyes and made him blind. While his way back home groping cattle, he fell in the well. He was worried that cattle will be lost and started crying. Cattle returned home but when Dhoumya did not see Upamanyu he was worried and went out in search of Upamanyu. He found him in the well, crying. Out of compassion he asked Upamanyu to chant vedic mantras in praise of Ashvini Kumars and by chanting those mantras Upamanyu got back his vision. Upamanyu found ways to satisfy his hunger but he never disobeyed commands of Dhoumya and hence Dhoumya was very pleased with him and blessed him with highest knowledge and all good fortune.
Such are the gurus who treat their disciples as their own children and then take their services to purify their hearts and then bless them with highest knowledge and fortune.
Wishing you all a very happy Gurupournima.