Mahabharata Shalya Parva Chapter 39

Mahabharata Shalya Parva (Gada Yuddha Parva) Chapter 39 Vaishampayana said, "Having passed one night more, Rama, having the plough for his weapon, worshipped the dwellers of that tirtha and showed his regard for Mankanaka. Having given wealth unto the Brahmanas, and passed the night there, the hero having the plough for his weapon was worshipped by the Munis. Rising up in the morning, he took leave of all the ascetics, and having touched the sacred water, O Bharata, set out quickly for other tirthas. Baladeva then went to the tirtha known by the name of Usanas. It is also called Kapalamochana. Formerly, Rama (the son of Dasaratha) slew a Rakshasa and hurled his head to a great distance. That head, O king, fell upon the thigh of a great sage named Mahodara and stuck to it. Bathing in this tirtha, the great Rishi became freed from the burthen. The high-souled Kavi (Sukra) had performed his ascetic penances there. It was there that the whole science of politics and morals (that goes by Sukra's name) appeared to him by inward light. While residing there, Sukra meditated upon the war of the Daityas and the Danavas (with the gods). Arrived at that foremost of tirthas, Baladeva, O king, duly made presents unto the high-souled Brahmanas." Janamejaya said, "Why is it called Kapalamochana, where the great Muni became freed (from the Rakshasa's head)? For what reason and how did that head stick unto him?" Vaishampayana said, "Formerly, O tiger among kings, the high-souled Rama (the son of Dasaratha) lived (for some time) in the forest of Dandaka, from desire of slaying the Rakshasas. At Janasthana he cut off the head of a wicked-souled Rakshasa with a razor-headed shaft of great sharpness. That head fell in the deep forest. That head, coursing at will (through the welkin) fell upon the thigh of Mahodara while the latter was wandering through the woods. Piercing his thigh, O king, it stuck to it and remained there. In consequence of that head thus sticking to his thigh, the Brahmana (Mahodara) of great wisdom could not (with ease) proceed to tirthas and other sacred spots. Afflicted with great pain and with putrid matter flowing from his thigh, he went to all the tirthas of the Earth (one after another), as heard by us. He went to all the rivers and to the ocean also. (Not finding any relief) the great ascetic spoke of his sufferings to many Rishis of cleansed souls about his having bathed in all the tirthas without having found the relief he sought. That foremost of Brahmanas then heard from those sages words of high import about this foremost of tirthas situate on the Sarasvati, and known by the name of Usanasa, which was represented as competent to cleanse from every sin and as an excellent spot for attaining to (ascetic) success. That Brahmana, then, repairing to that Usanasa tirtha, bathed in its waters. Upon this, the Rakshasa's head, leaving the thigh, fell into the water. Freed from that (dead) head, the Rishi felt great happiness. As regards the head itself, it was lost in the waters.