Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 2
Sanskrit:
देवानां शुद्धसत्त्वानामृषीणां चामलात्मनाम् ।भक्तिर्मुकुन्दचरणे न प्रायेणोपजायते ॥ २ ॥
ITRANS:
devānāṁ śuddha-sattvānāmṛṣīṇāṁ cāmalātmanāmbhaktir mukunda-caraṇena prāyeṇopajāyate
Translation:
Demigods situated in the mode of goodness and great saints cleansed of the dirt of material enjoyment hardly ever render pure devotional service at the lotus feet of Mukunda. [Therefore how could Vṛtrāsura have become such a great devotee?]