Valmiki Ramayana Yuddha Kanda Sarga 25 Shloka 10

ततः there upon, तौ both of them, शुकसारणौ Suka and Saarana, आचिन्त्यम् that which cannot be conceived, रोमहर्षणम् that which caused raising of hair (stunned), तत् that, वानरम् Vanara's, सैन्यम् army, सङ् ख्यातुम् to count the numbers, तदा that time, नाध्यगच्छेताम् whose number could not be known (not conceived).

There upon both Suka and Saarana did not know the numbers (of the army) at that time as they could not count the numbers from there as it could not be conceived. It caused the raising of their hair.