Valmiki Ramayana Kishkindha Kanda Sarga 41 Shloka 46
Original Shlokaराजधानी यमस्यैषा कष्टेन तमसा वृता। एतावदेव युष्माभिर्वीरा वानरपुङ्गवाः।।4.41.46।। शक्यं विचेतुं गन्तुं वा नातो गतिमतां गतिः।
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वीरा: वानरपुङ्गवाः heroic monkeys, एषा that is, यमस्य राजधानी capital of Yama, कष्टेन with pitch, तमसा darkness, आवृता covered, युष्माभिः by you folks, एतावदेव till there only, विचेतुम् to search, गन्तुं वा or to go, शक्यम् possible, अतः beyond this place, गतिमताम् for those who advance, गतिः path, न not.
'O heroic monkeys that is the capital of Yama (god of death) covered with pitch darkness. You can go till that place. Beyond, there is no access for the earthlings.