Srimad-Bhagavatam: Canto 7 - Chapter 1 - Verse 20
कथं तस्मिन् भगवति दुरवग्राह्यधामनि ।पश्यतां सर्वलोकानां लयमीयतुरञ्जसा ॥ २० ॥
kathaṁ tasmin bhagavatiduravagrāhya-dhāmanipaśyatāṁ sarva-lokānāṁlayam īyatur añjasā
How was it possible for Śiśupāla and Dantavakra, in the presence of many exalted persons, to enter very easily into the body of Kṛṣṇa, whose nature is difficult to attain?
Śiśupāla and Dantavakra were formerly Jaya and Vijaya, the doorkeepers of Vaikuṇṭha. Merging into the body of Kṛṣṇa was not their final destination. For some time they remained merged, and later they received the liberations of sārūpya and sālokya, living on the same planet as the Lord in the same bodily form. The śāstras give evidence that if one blasphemes the Supreme Lord, his punishment is to remain in hellish life for many millions of years more than one suffers by killing many brāhmaṇas. Śiśupāla, however, instead of entering hellish life, immediately and very easily received sāyujya-mukti. That such a privilege had been offered to Śiśupāla was not merely a story. Everyone saw it happen; there was no scarcity of evidence. How did it happen? Mahārāja Yudhiṣṭhira was very much surprised.