Srimad Bhagavatam Canto 06, Chapter 18, Text 54

SB 6.18.54

samvatsaram pumsavanam
 vratam etad aviplutam
dharayisyasi cet tubhyam
 sakra-ha bhavita sutah
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Kasyapa Muni continued: If you perform this ceremony called pumsavana, adhering to the vow with faith for at least one year, you will give birth to a son destined to kill Indra. But if there is any discrepancy in the discharge of this vow, the son will be a friend to Indra.
Srimad Bhagavatam Canto 06, Chapter 18, Text 53
Srimad Bhagavatam Canto 06, Chapter 18, Text 55