Valmiki Ramayana Sundara Kanda Sarga 10 Shloka 12

सः वानरर्षभः the bull among the vanaras, नागं यथा like a snake, निःश्वसन्तम् breathing heavily, रावणम् Ravana, आसाद्य having reached, परमोद्विग्नः very frightened, सुभीतवत् like a person in terrible fear, अपासर्पत् drew away

Having reached Ravana, who was breathing (hissing) like a snake, Hanuman was for a moment frightened and drew away from him as if he was put to terrible fear.