aprajasya manoh purvam
vasistho bhagavan kila
prajartham akarod vibhuh
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Manu at first had no sons. Therefore, in order to get a son for him, the great saint Vasistha, who was very powerful in spiritual knowledge, performed a sacrifice to satisfy the demigods Mitra and Varuna.