Srimad Bhagvata Mahapurana: Book 9: Chapter 9: Verses 1-13
The story of King Bhagiratha: the descent of the holy Ganga
Sri Suka resumed: (Even like Sagara, who renounced his kingdom,)
Amsuman too (abdicated the throne in favour of his son and) practised asceticism for a long time with a desire to bring down (from heaven) the holy Ganga, but did not succeed. Then, in course of time he passed away. His son, Dilipa (too) was unsuccessful like him and met his death (in the course of his efforts). Dilipa's son was Bhagiratha and he (too) performed very great Tapas. The goddess (presiding over the holy river) got pleased (with his devotion) and revealed herself (in person) to him, saying; "I am prepared to confer a boon on you." Thus spoken to (by the goddess), the king (Bhagiratha), bent low (with humility) and submitted (to her) his object (in carrying on his austerities). Somebody must sustain my force (even) as I descend on earth. Piercing the (hard) crust of the earth, otherwise, 0 protector of men. I shall penetrate into Rasatala (the sixth subterranean sphere from above). Further, I am not inclined to go down to the terrestrial region; (for) people would wash away their sin into me. Where shall I wash off that sin (myself), 0 Bhagiratha? Let some thought be bestowed on this point.
King Bhagiratha submitted : Pious souls who have given up attachment to actions as
well as to their fruit, are pure of mind, (nay,) foremost among the knowers of Brahma and contact of
capable of purifying the (whole) world, shall wash off your
(in their heart) dwells Sri Hari, the their body (while bathing in your water); for in Destroyer of sins. Again, Lord Rudra, the (very) Self of embodied souls---into whom this universe is woven, warp and woof, as a piece of cloth into its threads-will bear your force. Having spoken thus, the said king (Bhagiratha) proceeded to propitiate Lord Siva by means of his austerities. (And) after a very short time, 0 Pariksit, the Lord (Siva) became highly pleased (with him). Having accepted the king's prayer, saying, "So be it!" Lord Siva, the Friend of the whole world, cautiously bore (on His head) the Ganga, whose water stands hallowed by (the touch of) Sri Hari's foot. The said royal sage, Bhagiratha, led the Ganga (which is capable of purifying the whole universe) to the spot where the bodies of his great granduncles lay reduced to ashes. (Closely) following Bhagiratha, who rode in advance in a chariot swift as the wind, and hallowing the lands on its way, the Ganga bathed the burnt-up sons of Sagara. Though destroyed by their own offensive behaviour towards a (holy) Brahmana, the sons of Sagara ascended to heaven through the mere touch of its waters with their ashes alone! When the (aforesaid) sons of Sagara rose to heaven through the (mere) contact of its waters with their body converted into ashes, what need be said about them who make use of this heavenly stream with reverence (everyday), having taken a vow to this effect?