Mbh 3.307.1b

And thus did that child endued with great splendour and immeasurable prowess became the son of the charioteer, and came to be known as Vasusena and Vrisha. And Pritha learnt through spies that her own son clad in celestial mail was growing up amongst the Angas as the eldest son of a charioteer (Adhiratha). And seeing that in process of time his son had grown up, Adhiratha sent him to the city named after the elephant. And there Karna put up with Drona, for the purpose of learning arms. And that powerful youth contracted a friendship with Duryodhana. And having acquired all the four kinds of weapons from Drona, Kripa, and Rama, he became famous in the world as a mighty bowman. And after having contracted a friendship with Dhritarashtra’s son, he became intent on injuring the sons of Pritha. And he was always desirous of fighting with the high-souled Falguna. And, O king, ever since they first saw each other, Karna always used to challenge Arjuna, and Arjuna, on his part, used to challenge him. This, O foremost of kings, was without doubt, the secret known to the Sun, viz., begot by himself on Kunti, Karna was being reared in the race of the Sutas. And beholding him decked with his ear-rings and mail, Yudhishthira thought him to be unslayable in fight, and was exceedingly pained at it. And when, O foremost of monarchs, Karna after rising from the water, used at mid-day to worship the effulgent Surya with joined hands, the Brahmanas used to solicit him for wealth. And at that time there was nothing that he would not give away to the twice-born ones. And Indra, assuming the guise of a Brahmana, appeared before him (at such a time) and said, ‘Give me!’ And thereupon Radha’s son replied unto him, ‘Thou art welcome!’”