This story is mentioned in Valmiki Ramayan – Balakanda – chapter 46 , after the defeat of Asura’s in Devasura battle, the Diti went to his husband Rishi Kashyap and said :
“Lord, your highly powerful sons, (devatas) have killed mine through extensive austerities. I am desirous of obtaining a mighty son capable of killing Indra.
Rishi Kahyap replied : “If you remain pure and clean, on completion of till thousand years you will give birth to a son through me who will be the lord of the three worlds”.
When the Kasyapa had left, Diti being delighted at the prospect of having son reached place called Kushaplava (कुशप्लव ), and performed intense austerities. During these years Lord Indra came there and served her by providing fire, Kusha grass, sticks for burning, water, fruits, roots and other things desired by her. ( Since Indra was related to Diti, as he was the son of her Sister Aditi )
When the thousand years were about to complete, and only ten years were left, Diti was overpowered by sleep, and she unknowingly slept in an impure posture due to which her head hairs touched her feets.
At that moment Indra founding out her posture as impure, he entered into her womb and with an oath to destroy her embryo. Being severed by Vajra having a hundred edges, the child in the womb started crying at high pitch which awakened Diti.
Indra said to the child :
मा रुदो मा रुदश्चेति गर्भं शक्रोऽभ्यभाषत।
बिभेद च महातेजा रुदन्तमपि वासव:।।1.46.20।।
Highly powerful Indra, even while saying to the crying embryo “Do not cry, Do not cry” severed it into seven pieces.
After coming out of womb, Indra said “I have seized the opportunity and severed into seven pieces the foetus who would have been a slayer of Indra. O Devi shoudn’t you forgive me”?
Diti Replied –
This disaster to the embryo has happened contrary to expectations. Hence, I seek a favour from you. Let these seven pieces become seven guardians of Maruta (मरुत ) or regions of the wind.
O Son, all these seven are my sons of celestial beauty.They will become presiding deities for Vata Skandha (वातस्कन्ध , divisions of wind ) and let them be well-known as Maruta wandering in the universe.
Divison of Maruta (मारुत् )
There name Maruta (मारुत् ) was derived based on the address done by Indra as मा रुदो मा रुद: (don’t cry, don’t cry )
There are seven Maruta ( मारुत्) or seven winds – आवह, प्रवह, संवह , उद्वह, विवह, परिवह, and परावहः
1. आवह – winds assigned to the BhramLoka ब्रह्म्लोक
2. प्रवह – winds assigned to the Indraloka इन्द्रलोक
3. संवह – winds assigned to the Earth भूर्लोक
Rest four (उद्वह, विवह, परिवह, and परावहः) resides in four directions