Lord Ram once visited Sage Atri ,with his wife Sita and brother Lakshman .At his hermitage ,Sita met Anasuya ,who was the wife of sage Atri as described in the Valmiki Ramayana.
When Anasuya asks Sita about her marriage with Ram,and the most popular Sita Swayamvar,then Sita narrates the story of her birth and her swayamvar .
NARRATION OF THE STORY
Sita narrates the story as follows:-
There is a king in Mithila ,Janaka by name . He is heroic and conversant with righteousness .Once when he was ploughing the land for performing a sacrifice ,it is said that I emerged from the land by breaking it . As he had no children ,he lifted me up ,placing me on his lap and suddenly Janaka heard the following words from the sky: 'O king, she is an incomparable divine being. By right she is your daughter'. And then I was brought up by affection and love of my queen mother. When I attained a marriageable age ,he was immersed in sorrow like a men who has lost all his wealth . Janaka ,the ruler of earth knew that I was not born from a mothers womb ,and could not find a suitable husband for me ,and at last the wise king arrived at the decision to performing a swayamvar for me.
Varuna ,out of affection had given Janaka, an excellent bow with inexhaustible arrows .Because of its weight, no human could move that bow despite great effort nor were the kings capable of bending it even in their dreams.
The truthful king Janaka declared that the one who would lift the bow and string it ,his daughter would be married to him . Beholding the mighty bow resembling a mountain in weight, the kings, unable to lift it, paid their homage and left.
Now Sita narrates the arrival of Rama :
After a long time, resplendent Rama whose prowess was truth, arrived along with his brother Lakshman and sage Viswamitra in order to witness the sacrifice. 'Both these sons of king Dasaratha wish to see the bow. Show the divine bow to prince Rama',said Vishwamitra. At the words of that ascetic, my father brought out the bow. Mighty and valiant Rama bent it and strung it and drew it in the twinkling of an eye. When Rama was stringing the bow with force, it was broken into two in the middle and fell down with a dreadful sound like that of thunder.
When Janaka offered Rama to marry Sita ,Rama denied and asked for prior opinion of his father Dasrath,king of Ayodhya. Then Sitaji was married to Ramji on the accord of both Janaka and Dasrath.
Sitaji says :
In this way my father bestowed me on Rama in that swayamvara and I remain devoted to my husband who is the foremost among the valiant in treading the path of righteousness.
Sources : Ayodhyakand , Sarga 118,Valmiki Ramayana