Jarasandha, the ruler of Magadha, having by conciliation and honours obtained Karna for a friend, had challenged all the Kshatriyas of the world, except the Kauravas and the Yadavas(8,8)
Hearing of the fame of Karna's might, the ruler of the Magadhas, king Jarasandha, challenged him to a single combat. Both conversant with the celestial weapons, a fierce battle took place between them in which they struck each other with diverse kinds of arms. At last when their arrows were exhausted and bows and swords were broken and they both became careless, they began, possessed of might as they were, to fight with bare arms. While engaged with him in mortal combat with bare arms, Karna was about to sever the two portions of his antagonist's body that had been united together by Jara. The king (of Magadha), then after feeling himself very much pained, cast off all desire of hostility and addressed Karna, saying, 'I am gratified.' From friendship he then gave unto Karna the town Malini. Before this, that tiger among men and subjugator of all foes (viz., Karna) had been king of the Angasonly, but from that time the grinder of hostile forces began to rule over Champa also, agreeably to the wishes of Duryodhana (12,5)
Karna's promise to anyone who will discover the location of Arjuna for him in Kurukshetra War:- I shall give a hundred female slaves, with golden collars, belonging to the country of the Magadhas, and of very youthful age. (8,38).