Valmiki Ramayana Kishkindha Kanda Sarga 35 Shloka 17
Original Shlokaते न शक्या रणे हन्तुमसहायेन लक्ष्मण। रावणः क्रूरकर्मा च सुग्रीवेण विशेषतः।।4.35.17।।
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लक्ष्मण O Lakshmana, ते those, क्रूरकर्मा evildoer, रावणः Ravana, असहायेन without any help, रणे in war, हन्तुम् to kill, न शक्याः not possible, सुग्रीवेण by Sugriva, विशेषतः specially.
'O Lakshmana moreover it is not possible for Sugriva alone to kill the wicked Ravana without the help of the monkeys.