Mbh 12.3.1a

"'Narada said, "That tiger of Bhrigu's race (viz., Rama), was well-pleased with the might of Karna's arms, his affection (for him), his self-restraint, and the services he did unto his preceptor. Observant of ascetic penances, Rama cheerfully communicated, with due forms, unto his penance-observing disciple, everything about the Brahma weapon with the mantras for withdrawing it. Having acquired a knowledge of that weapon, Karna began to pass his days happily in Bhrigu's retreat, and endued with wonderful prowess, he devoted himself with great ardour to the science of weapons. One day Rama of great intelligence, while roving with Karna in the vicinity of his retreat, felt very weak in consequence of the fasts he had undergone. From affection begotten by confidence, the tired son of Jamadagni placing his head on Karna's lap, slept soundly. While his preceptor was thus sleeping (with head) on his lap, a frightful worm, whose bite was very painful and which subsisted on phlegm and fat and flesh and blood, approached the presence of Karna. That blood-sucking worm, approaching Karna's thigh, began to pierce it. Through fear of (awaking) his preceptor, Karna became unable to either throw away or kill that worm. Though his limb was bored through by that worm, O Bharata, the son of Surya, lest his preceptor should awake, suffered it to do its pleasure. Though the pain was intolerable, Karna bore it with heroic patience, and continued to hold Bhrigu's son on his lap, without quivering in the least and without manifesting any sign of pain. When at last Karna's blood touched the body of Rama of great energy, the latter awoke and said these words in fear, 'Alas, I have been made impure! What is this that thou art doing. Tell me, casting off all fear, what is the truth of this matter!' Then Karna informed him of that worm's bite. Rama saw that worm which resembled a hog in shape.