Valmiki Ramayana Bala Kanda Sarga 26 Shloka 22
Original Shlokaवध्यतां तावदेवैषा पुरा सन्ध्या प्रवर्तते।।1.26.22।। रक्षांसि सन्ध्याकालेषु दुर्धर्षाणि भवन्ति वै।
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सन्ध्या Dusk, पुरा प्रवर्तते is approaching, तावदेव before that time, एषा वध्यताम् she may be killed, सन्ध्याकालेषु during evening time, रक्षांसि rakshasas, दुर्धर्षाणि भवन्ति वै become unassailable.
Kill her, for dusk is fast approaching. During evening the strength of rakshasas tends to increase and they become unassailable".