Srimad Bhagvata Mahapurana: Book 6: Chapter 12: Verses 27-35
The demon, who was possessed of inordinate strength and extraordinary prowess and whose gigantic form was exceedingly tall, (now) stretched his lower jaw to the ground and extended the upper one to heaven; as though devouring (all) the three worlds (heaven, earth and the intervening space) with his mouth, deep as the sky, tongue fearful as the serpent's and teeth fierce as Death, nay, shaking mountains with (great) violence and pounding the earth under his feet like a huge mountain walking about, he went up to Indra (who was armed with his thunderbolt) and swallowed him with Airavata (that carried him on its back) even as a python would swallow an elephant. Seeing him devoured by Vrtra, the gods along with the lords of creation (Brahma and others) and eminent sages were seized with despair and exclaimed, "Ah, what a pity I" Though swallowed by Vrtra (the chief of the demons), and reaching his stomach, Indra did not die, protected as he was by Lord Narayana (the Supreme Person in the form of the armour-like Narayana-Kavaca) as well as by mystical powers and the powers of conjuration.
Ripping up his belly with his thunderbolt, and coming out (in this way), the powerful Indra (the slayer of the demon Bala) with (great) vigour lopped off the enemy's head like the top of a mountain. Though revolving with a quick speed, and cutting on all sides, the thunderbolt (of Indra) felled the neck of Vrtra in as many (three hundred and sixty) days as are taken by the northward and southward marches of (the sun and other) heavenly bodies, at the time appointed for the death of the demon. At that time drums sounded with a loud noise and the Gandharvas and Siddhas along with hosts of eminent sages joyously showered flowers on him, glorifying him with sacred hymns celebrating the prowess of the slayer of Vrtra. Issuing forth from the body of Vrtra in the form of an effulgence, O Pariksit (a chastiser of foes), the soul of Vrtra entered and merged into the Lord (who is beyond all the material worlds, while all the people (present there) looked on (with wonder).
Thus ends the twelfth discourse entitled "Vrtra slain," in Book Six of the great and glorious Bhagavata-Purana, otherwise known as the Paramahamsa-Samhita.