Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 68

Sanskrit:

आत्मानमिह सञ्जातं जानन्प्राग् विष्णुना हतम् । महासुरं कालनेमिं यदुभि: स व्यरुध्यत ॥ ६८ ॥

ITRANS:

ātmānam iha sañjātaṁjānan prāg viṣṇunā hatammahāsuraṁ kālanemiṁyadubhiḥ sa vyarudhyata

Translation:

In his previous birth, Kaṁsa had been a great demon named Kālanemi and been killed by Viṣṇu. Upon learning this information from Nārada, Kaṁsa became envious of everyone connected with the Yadu dynasty.

Purport:

Persons who are demons, enemies of the Supreme Personality of Godhead, are called asuras. As stated in Bhagavad-gītā, the asuras, because of their enmity toward the Supreme Personality of Godhead, take birth after birth in asura families and therefore glide down to the darkest hellish regions.