Valmiki Ramayana Kishkindha Kanda Sarga 42 Shloka 52
Original Shlokaअधिगम्य तु वैदेहीं निलयं रावणस्य च। अस्त पर्वतमासाद्य पूर्णे मासे निवर्तत।।4.42.52।।
1 source checked for this shloka
वैदेहीम् Vaidehi, रावणस्य Ravana's, निलयं च abode, अधिगम्य after finding out, अस्तं पर्वतम् Astha mountain, आसाद्य after reaching, मासे a month, पूर्णे in completion of, निवर्तत you may return
'Go and find out Sita and the abode of Ravana. After this, return to the mountain behind which the Sun sets and come back on completion of a month.