Mbh 9.39.3a

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. Mahodara then, O king, freed from the Rakshasa's head, cheerfully returned, with cleansed soul and all his sins washed away, to his asylum after achieving success. The great ascetic thus freed, after returning to his sacred asylum, spoke of what had happened to those Rishis of cleansed souls.