Here is a story from the Adi Parvan of the Mahabharata:
Once Drona, a Brahmana who taught archery to the
Kuru princes, was approached by Ekalavya, a forestdwelling
nishada (a hunting community). When Drona,
who knew the dharma, refused to have him as his pupil,
Ekalavya returned to the forest, prepared an image of
Drona out of clay, and treating it as his teacher, began
to practise on his own. In due course, he acquired
great skill in archery. One day, the Kuru princes went
hunting and their dog, wandering in the woods, came
upon Ekalavya. When the dog smelt the dark nishada
wrapped in black deer skin, his body caked with dirt,
it began to bark. Annoyed, Ekalavya shot seven arrows
into its mouth. When the dog returned to the Pandavas,
they were amazed at this superb display of archery.
They tracked down Ekalavya, who introduced himself
as a pupil of Drona.
Drona had once told his favourite student Arjuna,
that he would be unrivalled amongst his pupils. Arjuna
now reminded Drona about this. Drona approached
Ekalavya, who immediately acknowledged and
honoured him as his teacher. When Drona demanded
his right thumb as his fee, Ekalavya unhesitatingly cut
it off and offered it. But thereafter, when he shot with
his remaining fingers, he was no longer as fast as he
had been before. Thus, Drona kept his word: no one
was better than Arjuna.