Mahabharata 12.2.18

12.2.18

sa tatreṣvastramakarodbhṛguśreṣṭhādyathāvidhi |
priyaścābhavadatyarthaṃ devagandharvarakṣasām || 18 ||

I pray that all the masters of the science of weapons may, through thy grace, regard me as one accomplished in weapons!' Thus addressed by him, Drona, from partiality for Phalguna, as also from his knowledge of the wickedness of Karna, said, 'None but a Brahmana, who has duly observed all vows, should be acquainted with the Brahma weapon, or a Kshatriya that has practised austere penances, and no other.' When Drona had answered thus, Karna, having worshipped him, obtained his leave, and proceeded without delay to Rama then residing on the Mahendra mountains. Approaching Rama, he bent his head unto him and said, 'I am a Brahmana of Bhrigu's race.' This procured honour for him. With this knowledge about his birth and family, Rama received him kindly and said, 'Thou art welcome!' at which Karna became highly glad. While residing on the Mahendra mountains that resembled heaven itself, Karna met and mixed with many Gandharvas, Yakshas, and gods. Residing there he acquired all the weapons duly, and became a great favourite of the gods, the Gandharvas, and the Rakshasas. One day he roved on the sea-coast by the side of that asylum. Indeed, Surya's son, armed with bow and sword, wandered alone, While thus employed, O Partha, he inadvertently slew, without witting it, the Homa cow of a certain utterer of Brahma who daily performed his Agnihotra rite. Knowing that he had perpetrated that act from inadvertence, he informed the Brahmana of it.