Go now, Mother. Tell your sons nothing. Fight me. I will kill Arjuna or Arjuna will kill me. That is my gift to you: your five sons will live. I will aim only for the one who is not your son by Kunti's womb—no, wait. Arjuna is my brother too. So I will fight him as a Kshatriya fights a Kshatriya. Let fate decide."
(Helping her rise, with a heavy heart) "Yes, Mother. The world will know. But not while I live. A curse from my Guru Parasurama already ensures I will forget my divine weapons at the critical moment. And a Brahmin's curse ensures my chariot wheel will sink in the earth. So the world will see Karna die. But you... you will remember that Karna did not raise his hand against his mother, even when she deserved it. karna kunti sambad english translation
If I join the Pandavas now, the world will say: 'Karna is a traitor who changes sides for his mother's tears.' I cannot betray Duryodhana's salt. But... I will give you this promise: I will not kill any of your other sons. I will only fight Arjuna. Either he or I will die. You will still have five sons left, Mother. You will not be childless." Go now, Mother
(Weeping) "My child, I am not asking for your love. I am asking for your brothers—the Pandavas. They are your brothers. Arjuna is your brother. Do not kill them in this war. Join them, Karna. Let the truth be known. The throne of Hastinapura is yours by right as the eldest." I will kill Arjuna or Arjuna will kill me
Here is the English translation of the (the dialogue between Karna and Kunti), a pivotal episode from the Udyoga Parva (Book of Effort) of the Mahabharata.