Sagar's names is mentioned in Vaalmeeki Raamaayan in Baal Kaand, when Raaja Janak's Kul Gury Shataanand  (Gautam Muni and Ahilyaa's son) tells the story of Gangaa to Raam. This story comes in Bhaagvat Puraan also while telling the history of Soorya Vansh.

There was a king named Sagar in Soorya Vansh. He was the son of Raajaa Baahuk. Enemies took the kingdom of Baahuk so he went to forest along with his wife. When he got old there and died, his wife wanted to be Satee with him, but Maharshi Autva stopped her doing so, because she was pregnant. When Raajaa's other wives came to know about this they tried to give her poison (Gar) with food but it did not affect the child. Rather the child was born with "Gar", that is why his name was Sa-Gar (Sa means with, Gar means poison; means "with poison"). Later he became Chakravartee Samraat.

He had two wives but he had no son, so he went to his Kul Guru along with his wives and asked him for sons. Guru said - "You will have one son from your one wife and 60,000 from another wife, but your family line will grow only through that one son. so ask your wives who wants one son  and who wants 60,000 sons." His one wife Keshinee got one son and his another wife asked for 60,000 sons. At appropriate time both produced these sons. 

His one son's name was Asmanjas. Asmanjas was a Yogee in his previous life and he remembered everything. He used to do such Karm which showed him insane. He used to throw little playing children in Sarayoo River and when they started drowning, he used to laugh. So he made people tired of himself. So Raajaa Sagar exiled him. Later he gave life back to those children whom he threw in Sarayoo River. When their parents saw their children back, they got very happy and surprised. Sagar also repented for his action. Asmanjas had one son named Anshumaan. Raajaa used to love him very much because he was very obedient to his grandfather.

Once Sagar did Ashwamedh Yagya, but Indra took his horse away . Sagar sent his 60,000 sons to find it out. They went and looked for it on Prithvi, they couldn't find it. They dug Prthvi to find their horse, but still they couldn't find it. Now without that horse the Yagya could not be completed so Sagar sent them again to dig every inch of Prithvi and find it out and bring it. This time they found it tied with a tree in an Aashram. There was a Yogee doing Tapasyaa with closed eyes nearby that tree.

They recognized that horse and shouted on that Yogee - "You stole our horse and you are sitting here with closed eyes. You are the thief of our horse." He was not an ordinary Yogee, He was Bhagvaan Kapil (son of Maharshi Kardam and Devhooti). In fact Indra tied that horse there. Hearing them shouting as He opened His eyes they all burned.

When they did not return for some time, Raajaa Sagar sent his grandson Anshumaan to find out about his uncles as well as horse. Anshumaan also arrived at Kapil Muni's Aashram following the lead left by his uncles. There he saw his horse as well as a heap of ashes. He understood that the ash was of his uncles' bodies. He politely introduced himself to Kapil Jee and prayed Him. Kapil Jee got very pleased and said - "Son, This horse is your grandfather's Yagya animal, take it." Anshumaan asked Him - "How my uncles will get Mukti now?" Kapil Muni said - "Your uncles will get Mukti only when Gangaa waters will come and touch these ashes."

Anshumaan got back to his grandfather and told everything. Raajaa completed his Yagya, then he handed over his kingdom to Anshumaan and left the body. Later Anshumaan did very intense Tap to bring Gangaa on Prithvi but could not succeed. His son was Dileep, he also did similar efforts but he also could not succeed. Dileep's son was Bhageerath. He was able to bring Gangaa on Prithvi and give Mukti to his great grandfathers.