SB 9.23.33

teṣāṁ tu ṣaṭ pradhānānāṁ
 pṛthuśravasa ātmajaḥ
dharmo nāmośanā tasya
 hayamedha-śatasya yāṭ
Synonyms: 
teṣām — out of so many sons; tu — but; ṣaṭ pradhānānām — of whom there were six foremost sons; pṛthuśravasaḥ — of Pṛthuśravā; ātmajaḥ — the son; dharmaḥ — Dharma; nāma — by the name; uśanā — Uśanā; tasya — his; hayamedha-śatasya — of one hundred aśvamedha sacrifices; yāṭ — he was the performer.
Translation: 
Among these many sons, six were the foremost, such as Pṛthuśravā and Pṛthukīrti. The son of Pṛthuśravā was known as Dharma, and his son was known as Uśanā. Uśanā was the performer of one hundred horse sacrifices.