Mbh 12.1.4b

Having placed him in a basket made of light materials, Kunti committed him to the current of Ganga. He who was regarded by the world as a Suta's child born of Radha, was really the eldest son of Kunti and, therefore, our uterine brother. Covetous of kingdom, alas, I have unwittingly caused that brother of mine to be slain. It is this that is burning my limbs like a fire burning a heap of cotton. The white-steeded Arjuna knew him not for a brother. Neither I, nor Bhima, nor the twins, knew him for such. He, however, of excellent bow, knew us (for his brothers). We have heard that on one occasion Pritha went to him for seeking our good and addressed him, saying, 'Thou art my son!' That illustrious hero, however, refused to obey Pritha's wishes. Subsequently, we are informed, he said unto his mother these words, 'I am unable to desert Duryodhana in battle! If I do so, it would be a dishonourable, cruel, and ungrateful act. If, yielding to thy wishes, I make peace with Yudhishthira, people will say that I am afraid of the white-steeded Arjuna. Having vanquished Arjuna with Kesava, therefore, in battle, I will subsequently make peace with Dharma's son.' Even these were his words as we have heard. Thus answered, Pritha once more addressed her son of broad chest and said, 'Fight Phalguna then, but spare my four other sons.' The intelligent Karna, with joined hands, then replied unto his trembling mother, saying, 'If I get thy four other sons even under my power, I will not slay them. Without doubt, O goddess, thou shalt continue to have five sons. If Karna be slain with Arjuna, thou shalt have five! If, on the other hand, Arjuna be slain, thou shalt have five, numbering me.' Desirous of the good of her children, his mother once more said unto him, 'Go, O Karna, do good unto those brothers of thine whose good thou always seekest.' Having said these words, Pritha took his leave and came back to her abode.