Valmiki Ramayana Bala Kanda Sarga 48 Shloka 19
Original Shlokaमुनिवेषं सहस्राक्षं विज्ञाय रघुनन्दन।1.48.19 मतिं चकार दुर्मेधा देवराजकुतूहलात्।।
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रघुनन्दन O Rama, दुर्मेधा: evilintentioned, मुनिवेषम् in the guise of ascetic, सहस्राक्षम् Indra, विज्ञाय knowing, देवराजकुतूहलात् out of inclination towards the king of celestials, मतिं चकार consented.
O Delight of the Raghus the evilintentioned Ahalya, inclined towards the king of the celestials and knowing him to be the thousandeyed Indra in the guise of the ascetic, consented for the union.