SB 7.1.16
sri-yudhisthira uvaca
aho aty-adbhutam hy etad
durlabhaikantinam api
vasudeve pare tattve
praptis caidyasya vidvisah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Maharaja Yudhisthira inquired: It is very wonderful that the demon Sisupala merged into the body of the Supreme Personality of Godhead even though extremely envious. This sayujya-mukti is impossible to attain even for great transcendentalists. How then did the enemy of the Lord attain it?
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There are two classes of transcendentalists — the jñanis and the bhaktas. The bhaktas do not aspire to merge into the existence of the Lord, but the jñanis do. Sisupala, however, was neither a jñani nor a bhakta, yet simply by envy of the Lord he attained an exalted position by merging into the Lord’s body. Certainly this was astonishing, and therefore Maharaja Yudhisthira inquired about the cause for the Lord’s mysterious mercy to Sisupala.