Valmiki Ramayana Sundara Kanda Sarga 15 Shloka 38
Original Shlokaतस्य संदिदिहे बुद्धिर्मुहुः सीतां निरीक्ष्य तु। आम्नायानामयोगेन विद्यां प्रशिथिलामिव।।5.15.38।।
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आम्नायानाम् of the great shastriclore, अयोगेन by not practising, प्रशिथिलाम् slackened, विद्यामिव like the knowledge, सीताम् Sita, निरीक्ष्य having observed, तस्य his, बुद्धिस्तु intellect also, मुहुः after seeing carefuly again and again, संदिदिहे doubted.
Hanuman did recognise Sita after looking at her carefully again and again even as one is able to make out knowledge of shastriclore forgotten with lapse of time and lack of practice.