Mbh 12.282.16

"'Bhishma continued, "Then the sin of Brahmanicide, O Yudhishthira, leaving the chief of the deities, proceeded to the abodes that were ordained for her at the Grandsire's command. It was thus, O ruler of men, that Indra had become afflicted by that dreadful sin (and it was thus that he got rid of her). With the Grandsire's permission Indra then resolved to perform a Horse-sacrifice. It is heard, O monarch, that Indra having been thus possessed by the sin of Brahmanicide afterwards became cleansed of her through that Sacrifice. Regaining his prosperity and slaying thousands of foes, great was the joy that Vasava obtained, O lord of Earth! From the blood of Vritra, O son of Pritha, were born high-crested cocks. For this reason, those fowls are unclean (as food) for the regenerate classes, and those ascetics that have undergone the rite of initiation. Under all circumstances, O king, do thou accomplish what is agreeable to the twice-born, for these, O monarch, are known as gods on earth. It was in this way, O thou of Kuru's race, that the mighty Asura Vritra was slain by Sakra of immeasurable energy by the aid of subtle intelligence and through the application of means. Thou also, O son of Kunti, unvanquished on earth, wilt become another Indra and the slayer of all thy foes. Those men who, on every Parva day, will recite this sacred narrative of Vritra in the midst of Brahmanas shall never be stained by any sin. I have now recited to thee one of the greatest and most wonderful feats of Indra connected with Vritra. What else dost thou wish to hear?"'"