Srimad Bhagavatam Canto 09, Chapter 13, Text 06

SB 9.13.6

ity utsasarja svam deham
 nimir adhyatma-kovidah
mitra-varunayor jajñe
 urvasyam prapitamahah
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
After saying this, Maharaja Nimi, who was expert in the science of spiritual knowledge, gave up his body. Vasistha, the great-grandfather, gave up his body also, but through the semen discharged by Mitra and Varuna when they saw Urvasi, he was born again.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Mitra and Varuna chanced to meet Urvasi, the most beautiful prostitute of the heavenly kingdom, and they became lusty. Because they were great saints, they tried to control their lust, but they could not do so, and thus they discharged semen. This semen was kept carefully in a waterpot, and Vasistha was born from it.
Srimad Bhagavatam Canto 09, Chapter 13, Text 05
Srimad Bhagavatam Canto 09, Chapter 13, Text 07