Valmiki Ramayana Kishkindha Kanda Sarga 27 Shloka 39
Original Shlokaशरत्कालं प्रतीक्षस्व प्रावृट्कालोऽयमागतः। ततस्सराष्ट्रं सगणं रावणं त्वं वधिष्यसि4.27.39।।
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अयम् this, प्रावृट्कालः rainy season, आगतः has set in, शरत्कालम् autumn, प्रतीक्षस्व you may wait for, ततः then, सराष्ट्रम् along with his country, सगणम् along with his troops, त्वं you, रावणम् Ravana, वधिष्यसि will slay.
'This is the rainy season.Wait for autumn.Thereafter you will destroy Ravana along with his kingdom and clan.