Srimad Bhagvata Mahapurana: Book 6: Chapter 13: Verses 1-13
Sri Suka began again : Vrtra having (thus) been slain, (all) the three worlds with their guardian deities, excepting, of course, Indra, O munificent Pariksit, were immediately rid of anxiety and felt gratified at heart. The gods and sages, manes and spirits, the demons and. the attendants of the gods (Gandharvas and others) returned to their (respective) realms of their own accord without taking leave of (Indra) and then Brahma (the creator), Siva, Indra and other (great) gods (too) dispersed. The king (Pariksit) said : I long to hear (from you). O (holy) sage, the cause of Indra's unhappiness. Why should there be agony to Indra (the chief of the gods) from something as a result of which the (other) gods felt gratified ? Sri Suka replied : Frightened with the prowess of the demon Vrtra all the gods along with the sages implored Indra to slay him. He, however, did not feel inclined to do so, afraid as he was of killing a Brahmana.
Indra said : The sin caused by the slaughter of Viswarupa has (since) been graciously shared by women, the earth, water and the trees. (But) how shall I (be able to) atone for (the sin proceeding from) the slaughter of Vrtra? Sri Suka continued : On hearing this the sages replied to the mighty Indra as follows:-"We shall get you to propitiate the Lord by means of a horse-sacrifice and all will be well with you. (Pray) too not be afraid. Having worshipped Lord Narayana, the Supreme Spirit, the Inner Controller and Ruler of the universe, through a horse-sacrifice you will be absolved even of the sin proceeding from the extermination of the (whole) world. Through the (very) utterance of His Name the murderer of a Brahmana, the slaughterer of a cow, the slayer of one's father, mother or preceptor and any other sinner, (nay,) one who eats the flesh of a dog and even a man of sinful birth are purified (at once). Propitiating Him with reverence through the great sacrifice, Aswamedha, which will be performed by us, you will not be touched by sin even after killing the (entire) mobile and immobile creation including the Brahmana race, much less by the sin that may follow from the subjugation of an evil-doer (like Vrtra)." Sri Suka went on : Thus urged by the Brahmanas, Indra killed Vrtra (his enemy). on his being (thus) slain, the sin of having killed a Brahmana came upon Indra. Due to (the consciousness of) that sin Indra suffered (untold) agony and no peace of mind returned to him (even for a moment ). Even (fortitude and other) virtues fail to bring relief to him who, while endowed with a sense of shame, has incurred obloquy. He saw the aforesaid sin running after him in human form, resembling that of a pariah woman suffering from consumption, clad in blood-stained clothes, her limbs trembling due to old age, and throwing about her grey hair, crying "Stop ! Stop !!" and befouling (the air of) the road with her breaths stinking like (rotten) fish.