Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 7
Sanskrit:
अत्र न: संशयो भूयाञ्छ्रोतुं कौतूहलं प्रभो ।य: पौरुषेण समरे सहस्राक्षमतोषयत् ॥ ७ ॥
ITRANS:
atra naḥ saṁśayo bhūyāñchrotuṁ kautūhalaṁ prabhoyaḥ pauruṣeṇa samaresahasrākṣam atoṣayat
Translation:
My dear lord, Śukadeva Gosvāmī, although Vṛtrāsura was a sinful demon, he showed the prowess of a most exalted kṣatriya and satisfied Lord Indra in battle. How could such a demon be a great devotee of Lord Kṛṣṇa? These contradictions have caused me great doubt, and they have made me eager to hear of this from you.