Srimad Bhagavatam Canto 10, Chapter 36, Text 19

 SB 10.36.19

nivarito naradena
 tat-sutau mrtyum atmanah
jñatva loha-mayaih pasair
 babandha saha bharyaya
But Narada restrained Kamsa by reminding him that it was the two sons of Vasudeva who would cause his death. Kamsa then had Vasudeva and his wife shackled in iron chains.
Kamsa realized that there was no use in killing Vasudeva, since it was Vasudeva’s sons, Krsna and Balarama, who were to kill him. According to the acaryas, Narada also advised Kamsa that if he killed Vasudeva the two young boys might flee and that it was therefore better not to kill him. Rather, Narada recommended, Kamsa should bring Krsna and Balarama to Kamsa’s capital city, Mathura.
Srila Visvanatha Cakravarti points out that Narada did not act inimically toward the great devotees Vasudeva and Devaki when he revealed this information to Kamsa. In fact, as explained in the Eleventh Canto, Vasudeva was grateful to Narada because he was arranging for Kamsa’s death at Krsna’s hands, and further arranging for Krsna to come and live in Mathura, where His loving father could associate with Him.
Srimad Bhagavatam Canto 10, Chapter 36, Text 18
Srimad Bhagavatam Canto 10, Chapter 36, Text 20