Ramayana
उत्पन्ना मैथिलकुले जनकस्य महात्मनः । सीतोत्पन्ना तु सीतेति मानुषैः पुनरुच्यते ॥ 7-17-43 ॥
she was born in the Maithila line, of the great-souled Janaka; and because she sprang up from the furrow (sita), she is called Sita by men.
english translation