Bhagavatapurana 9.13.18

9.13.18

tataḥ śīradhvajo jajñe yajñārthaṃ karṣato mahīm /
sītā śīrāgrato jātā tasmāt śīradhvajaḥ smṛtaḥ //

17. & 18. His son was Kritirata whose son was Maharoman;. his son Svarnaroman who begot Hrasvaroman; from him descended Seeradhvaja; when he ploughed the earth for the sake of sacrifice, Sita was obtained at the point of the ploughshare and hence he is known as Seeradhvaja.