Valmiki Ramayana Yuddha Kanda Sarga 50 Shloka 54
Original Shlokaतन्नविश्वसनीयंवोराक्षसानांरणाजिरे । एतेनैवोपमानेननित्यंजिह्माहिराक्षसाः ।।6.50.54।।
1 source checked for this shloka
तत् being so, रणाजिरे in war, वः them, राक्षसानाम् Rakshasas, नविश्वसनीयम् not to be trusted, एतेन because, उपमानेनैव by this experience, राक्षसाः of Rakshasa, नित्यम् ever, जिह्माःहि crooked.
"Being so you should not trust Rakshasas. By this experience you know that Rakshasas are crooked."