Vishnupurana 1.21.33

1.21.33

śakraṃ putro nihantā te yadi garbhaṃ śaracchatam //

Diti, having lost her children, propitiated Kaśyapa; and the best of ascetics, being pleased with her, promised her a boon; on which she prayed for a son of irresistible prowess and valour, who should destroy Indra. The excellent Muni granted his wife the great gift she had solicited, but with one condition: "You shall bear a son," he said, "who shall slay Indra, if with thoughts wholly pious, and person entirely pure, you carefully carry the babe in your womb for a hundred years." Having thus said, Kaśyapa departed; and the dame conceived, and during gestation assiduously observed the rules of mental and personal purity. When the king of the immortals, learnt that Diti bore a son destined for his destruction, he came to her, and attended upon her with the utmost humility, watching for an opportunity to disappoint her intention. At last, in the last year of the century, the opportunity occurred. Diti retired one night to rest without performing the prescribed ablution of her feet, and fell asleep; on which the thunderer divided with his thunderbolt the embryo in her womb into seven portions. The child, thus mutilated, cried bitterly; and Indra repeatedly attempted to console and silence it, but in vain: on which the god, being incensed, again divided each of the seven portions into seven, and thus formed the swift-moving deities called Márutas (winds). They derived this appellation from the words with which Indra had addressed them (Má rodíh, 'Weep not'); and they became forty-nine subordinate divinities, the associates of the wielder of the thunderbolt.