Mbh 9.43.3a

Vaishampayana said, "Listen to that history, O ruler of men! Hear of those occurrences as they happened! Hear how Vasava, in days of yore, broke his treaty with Namuchi! The Asura Namuchi, from fear of Vasava, had entered a ray of the Sun. Indra then made friends with Namuchi and entered into a covenant with him, saying, 'O foremost of Asuras, I shall not slay thee, O friend, with anything that is wet or with anything that is dry! I shall not slay thee in the night or in the day! I swear this to thee by truth.' Having made this covenant, the lord Indra one day beheld a fog. He then, O king, cut off Namuchi's head, using the foam of water (as his weapon). The severed head of Namuchi thereupon pursued Indra from behind, saying unto him from a near point these words, 'O slayer of a friend, O wretch!' Urged on incessantly by that head, Indra repaired to the Grandsire and informed him, in grief, of what had occurred. The Supreme Lord of the universe said unto him, 'Performing a sacrifice, bathe with due rites, O chief of the celestials, in Aruna, that tirtha which saveth from the fear of sin! The water of that river, O Shakra, hath been made sacred by the Munis! Formerly the presence of that river at its site was concealed. The divine Sarasvati repaired to the Aruna, and flooded it with her waters. This confluence of Sarasvati and Aruna is highly sacred! Thither, O chief of the celestials, perform a sacrifice! Give away gifts in profusion! Performing thy ablutions there, thou shall be freed from thy sin.' Thus addressed, Shakra, at these words of Brahma, O Janamejaya, performed in that abode of Sarasvati diverse sacrifices. Giving away many gifts and bathing in that tirtha, he of a hundred sacrifices, the piercer of Vala, duly performed certain sacrifices and then plunged in the Aruna. He became freed from the sin arising out of the slaughter of a Brahmana. The lord of heaven then returned to heaven with a joyful heart.