SB 9.1.13

aprajasya manoḥ pūrvaṁ
 vasiṣṭho bhagavān kila
mitrā-varuṇayor iṣṭiṁ
 prajārtham akarod vibhuḥ
Synonyms: 
aprajasya — of he who had no son; manoḥ — of Manu; pūrvam — formerly; vasiṣṭhaḥ — the great saint Vasiṣṭha; bhagavān — powerful; kila — indeed; mitrā-varuṇayoḥ — of the demigods named Mitra and Varuṇa; iṣṭim — a sacrifice; prajā-artham — for the sake of getting sons; akarot — executed; vibhuḥ — the great person.
Translation: 
Manu at first had no sons. Therefore, in order to get a son for him, the great saint Vasiṣṭha, who was very powerful in spiritual knowledge, performed a sacrifice to satisfy the demigods Mitra and Varuṇa.