Valmiki Ramayana Yuddha Kanda Sarga 91 Shloka 49
Original Shlokaकुबेरेणस्वयंस्वप्नेयद्दत्तममितात्मना ।।6.91.49।। दुर्जयंदुर्विषह्यं च सेन्द्रैरपिसुरासुरैः ।
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यत् that arrow, अमितात्मना god of wealth, कुबेरेण by Kubera, स्वयम् himself, स्वप्ने to him in dream, दत्तम् gifted, सेन्रन्दैः, सुरासुरैरपि even for suras and Asuras, दुर्जयम् difficult to win, दुर्विषह्यं च or to resist
That arrow was gifted to Lakshmana in dream by Kubera, the god of wealth and even Suras or Asuras cannot resist it and is difficult to win.