Valmiki Ramayana Sundara Kanda Sarga 37 Shloka 23
Original Shlokaत्वद्दर्शनकृतोत्साहमाश्रमस्थं महाबलम्। पुरन्दरमिवासीनं नागराजस्य मूर्धनि।।5.37.23।।
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त्वद्दर्शनकृतोत्साहम् enthused in seeing you, आश्रमस्थम् staying in his asram, महाबलम् mighty, नागराजस्य of the king of elephants Airavatam, मूर्धनि on the back, आसीनम् seated, पुरन्दरमिव like the destroyer of citadels (Indra).
Mighty Rama staying in asram ill be enthused to see you seated on my back like Lord Purandara (Indra) seated on the back of the lord of elephants (Airavata)