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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
अप्रजस्य मनोः पूर्वं वसिष्ठो भगवान् किल । मित्रावरुणयोरिष्टिं प्रजार्थमकरोत्प्रभुः
aprajasya manoḥ pūrvaṃ vasiṣṭho bhagavān kila | mitrāvaruṇayoriṣṭiṃ prajārthamakarotprabhuḥ
SB 9.1.13