Once the great saint Nārada approached Kaṁsa and informed him of how the demoniac persons who were a great burden on the earth were going to be killed. Thus Kaṁsa was placed into great fear and doubt.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
एतत्कंसाय भगवाञ्छशंसाभ्येत्य नारदः । भूमेर्भारायमाणानां दैत्यानां च वधोद्यमम्
etatkaṃsāya bhagavāñchaśaṃsābhyetya nāradaḥ | bhūmerbhārāyamāṇānāṃ daityānāṃ ca vadhodyamam
SB 10.1.64