Mahabharata Adi Parva Chapter 111:2

Mahabharata Adi Parva Chapter 111:2 And after the birth of this child, the illustrious Tapana granted unto Pritha her maidenhood and ascended to heaven. And the princess of the Vrishni race beholding with sorrow that son born of her, reflected intently upon what was then the best for her to do. And from fear of her relatives she resolved to conceal that evidence of her folly. And she cast her offspring endued with great physical strength into the water. Then the well-known husband of Radha, of the Suta caste, took up the child thus cast into the water, and he and his wife brought him up as their own son. And Radha and her husband bestowed on him the name of Vasusena (born with wealth) because he was born with a natural armour and ear-rings. And endued as he was born with great strength, as he grew up, he became skilled in all weapons. Possessed of great energy, he used to adore the sun until his back was heated by his rays (i.e., from dawn to midday), and during the hours of worship, there was nothing on earth that the heroic and intelligent Vasusena would not give unto the Brahmanas. And Indra desirous of benefiting his own son Phalguni (Arjuna), assuming the form of a Brahmana, approached Vasusena on one occasion and begged of him his natural armour. Thus asked Karna took off his natural armour, and joining his hands in reverence gave it unto Indra in the guise of a Brahmana. And the chief of the celestials accepted the gift and was exceedingly gratified with Karna's liberality. He therefore, gave unto him a fine dart, saying, 'That one (and one only) among the celestials, the Asuras, men, the Gandharvas, the Nagas, and the Rakshasas, whom thou desirest to conquer, shall be certainly slain with this dart.' "The son of Surya was before this known by the name of Vasusena. But since he cut off his natural armour, he came to be called Karna (the cutter or peeler of his own cover).'"