Valmiki Ramayana Kishkindha Kanda Sarga 35 Shloka 16
Original Shlokaअहत्वा तांश्च दुर्धर्षान्राक्षसान्कामरूपिणः। न शक्यो रावणो हन्तुं येन सा मैथिली हृता।।4.35.16।।
1 source checked for this shloka
दुर्धर्षान् unassailable, कामरूपिणः who can assume any form at will, तान् them, राक्षसान् demons, अहत्वा without killing, येन by whomsoever, सा she, मैथिली Maithili, हृता stolen, रावण: Ravana, हन्तुम् to kill, न शक्यः not possible.
'It is not possible to kill Ravana who has stolen Sita without killing those unassailable demons who can assume any form at their will.