Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 18

Sanskrit:

अध्वर्युणात्तहविषा यजमानो विशाम्पते ।धिया विशुद्धया दध्यौ तथा प्रादुरभूद्धरि: ॥ १८ ॥

ITRANS:

adhvaryuṇātta-haviṣāyajamāno viśāmpatedhiyā viśuddhayā dadhyautathā prādurabhūd dhariḥ

Translation:

The great sage Maitreya said to Vidura: My dear Vidura, as soon as King Dakṣa offered the clarified butter with Yajur Veda mantras in sanctified meditation, Lord Viṣṇu appeared there in His original form as Nārāyaṇa.

Purport:

Lord Viṣṇu is all-pervading. Any devotee who, in sanctified meditation, following the regulative principles, chants the required mantras in service and in a devotional mood can see Viṣṇu. It is said in the Brahma-saṁhitā that a devotee whose eyes are anointed with the ointment of love of Godhead can see the Supreme Personality of Godhead always within his heart. Lord Śyāmasundara is so kind to His devotee.