Valmiki Ramayana Sundara Kanda Sarga 1 Shloka 75
Original Shlokaप्लवमानं समीक्ष्याथ भुजङ्गाः सागरालयाः। व्योम्नि तं कपिशार्दूलं सुपर्ण इति मेनिरे।।5.1.75।।
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अथ and then, व्योम्नि in the sky, प्लवमानम् coursing through, तं कपिशार्दूलम् that tiger among vanaras, समीक्ष्य seeing, सागरालयाः those in the abode of the sea, भुजङ्गाः serpents, सुपर्णः इति to be Garuda, मेनिरे thought.
Seeing Hanuman, the tiger among vanaras, coursing through the sky the serpents in their seaabode mistook him for Garuda.