In the Udyoga Parva of the Mahabharata, Krishna comes to Hastinapura on a mission of peace. After the talks, as he leaves the city, he takes Karna with him. King Dhritarashtra later asks what Krishna said after he “took Karna upon his car,” in the Ganguli translation.
In private, Krishna tells Karna that he is a son of Kunti, born before Yudhishthira. He offers him the throne. “Come, be a king,” he says. Yudhishthira would be his heir.