Valmiki Ramayana Aranya Kanda Sarga 11 Shloka 18
Original Shlokaतथैवाप्सरसः पञ्च निवसन्त्यो यथासुखम्।।3.11.18।। रमयन्ति तपोयोगान्मुनिं यौवनमास्थितम्।
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तथैव like that, अप्सरसः the apsaras, यथासुखम् happily निवसन्त्य: while living there, तपोयोगात् by the power of penance, यौवनम् youth, आस्थितम् attaining, मुनिम् sage, रमयन्ति are entertaining.
The apsaras lived happily with the sage who remained youthful by virtue of his penance.