Srimad-Bhagavatam: Canto 9 - Chapter 1 - Verse 18
Sanskrit:
तत: शीरध्वजो जज्ञे यज्ञार्थं कर्षतो महीम् ।सीता शीराग्रतो जाता तस्मात् शीरध्वज: स्मृत: ॥ १८ ॥
ITRANS:
tataḥ śīradhvajo jajñeyajñārthaṁ karṣato mahīmsītā śīrāgrato jātātasmāt śīradhvajaḥ smṛtaḥ
Translation:
From Hrasvaromā came a son named Śīradhvaja [also called Janaka]. When Śīradhvaja was plowing a field, from the front of his plow [śīra] appeared a daughter named Sītādevī, who later became the wife of Lord Rāmacandra. Thus he was known as Śīradhvaja.