Mahabharata
In the Mahabharata, Jambavan had killed a lion, who had acquired a gem called syamantaka from Prasena, after killing him. Krishna was suspected of killing Prasena for the jewel, so he tracked Prasena's steps until he learned that he had been killed by a lion, who had been killed by a bear. Krishna tracked Jambavan to his cave, and a fight ensued. The combat between Krishna and Jambavan ensued for 27/28 days (per Bhagavata Purana) and 21 days (per Vishnu Purana), after which Jambavan began to grow tired. Realising who Krishna was, Jambavan submitted. He gave Krishna the gem, and also presented him his daughter Jambavati, who became one of Krishna's wives.