Devibhagavatapurana 3.15.28

3.15.28

śrūyate hi purendreṇa māturgarbhagataḥ śiśuḥ |
kṛntitaḥ saptadhā paścātkṛtāste sapta saptadhā ||

I heard that, in days of yore, Indra entered into the womb of his pregnant step mother with a small thunderbolt in his hand and divided the foetus into seven parts with that weapon, again each of these seven into seven parts again, thus the forty nine Maruts were born in the Heavens.