SB 4.7.18
adhvaryunatta-havisa
yajamano visampate
dhiya visuddhaya dadhyau
tatha pradurabhud dharih
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The great sage Maitreya said to Vidura: My dear Vidura, as soon as King Daksa offered the clarified butter with Yajur Veda mantras in sanctified meditation, Lord Visnu appeared there in His original form as Narayana.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Lord Visnu 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 Visnu. It is said in the Brahma-samhita 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 Syamasundara is so kind to His devotee.