SB 9.13.18
tatah siradhvajo jajñe
yajñartham karsato mahim
sita siragrato jata
tasmat siradhvajah smrtah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
From Hrasvaroma came a son named Siradhvaja [also called Janaka]. When Siradhvaja was plowing a field, from the front of his plow [sira] appeared a daughter named Sitadevi, who later became the wife of Lord Ramacandra. Thus he was known as Siradhvaja.