SB 7.1.14-15

dṛṣṭvā mahādbhutaṁ rājā
 rājasūye mahā-kratau
vāsudeve bhagavati
 sāyujyaṁ cedibhū-bhujaḥ
tatrāsīnaṁ sura-ṛṣiṁ
 rājā pāṇḍu-sutaḥ kratau
papraccha vismita-manā
 munīnāṁ śṛṇvatām idam
Synonyms: 
dṛṣṭvā — after seeing; mahā-adbhutam — greatly wonderful; rājā — the King; rājasūye — called Rājasūya; mahā-kratau — at the great sacrifice; vāsudeve — into Vāsudeva; bhagavati — the Personality of Godhead; sāyujyam — merging; cedibhū-bhujaḥ — of Śiśupāla, the King of Cedi; tatra — there; āsīnam — seated; sura-ṛṣim — Nārada Muni; rājā — the King; pāṇḍu-sutaḥ — Yudhiṣṭhira, the son of Pāṇḍu; kratau — at the sacrifice; papraccha — asked; vismita-manāḥ — being struck with wonder; munīnām — in the presence of the sages; śṛṇvatām — listening; idam — this.
Translation: 
O King, at the Rājasūya sacrifice, Mahārāja Yudhiṣṭhira, the son of Mahārāja Pāṇḍu, personally saw Śiśupāla merge into the body of the Supreme Lord, Kṛṣṇa. Therefore, struck with wonder, he inquired about the reason for this from the great sage Nārada, who was seated there. While he inquired, all the sages present also heard him ask his question.