SB 7.1.16

śrī-yudhiṣṭhira uvāca
aho aty-adbhutaṁ hy etad
 durlabhaikāntinām api
vāsudeve pare tattve
 prāptiś caidyasya vidviṣaḥ
Word for word: 
śrī-yudhiṣṭhiraḥ uvāca — Mahārāja Yudhiṣṭhira said; aho — oh; ati-adbhutam — very wonderful; hi — certainly; etat — this; durlabha — difficult to attain; ekāntinām — for the transcendentalists; api — even; vāsudeve — in Vāsudeva; pare — the supreme; tattve — Absolute Truth; prāptiḥ — the attainment; caidyasya — of Śiśupāla; vidviṣaḥ — envious.
Translation: 
Mahārāja Yudhiṣṭhira inquired: It is very wonderful that the demon Śiśupāla merged into the body of the Supreme Personality of Godhead even though extremely envious. This sāyujya-mukti is impossible to attain even for great transcendentalists. How then did the enemy of the Lord attain it?
Purport: 

There are two classes of transcendentalists — the jñānīs and the bhaktas. The bhaktas do not aspire to merge into the existence of the Lord, but the jñānīs do. Śiśupāla, however, was neither a jñānī 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 Mahārāja Yudhiṣṭhira inquired about the cause for the Lord’s mysterious mercy to Śiśupāla.