Valmiki Ramayana Sundara Kanda Sarga 15 Shloka 39
Original Shlokaदुःखेन बुबुधे सीतां हनुमाननलङ्कृताम्। संस्कारेण यथा हीनां वाचमर्थान्तरं गताम्।।5.15.39।।
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हनुमान Hanuman, अनलङ्कृताम् devoid of decoration, संस्कारेण by virtue of previous practice, हीनाम् devoid, अर्थान्तरम् changing its meaning, गताम् interpreted thus, वाचम् यथा like the words, सीताम् Sita, दुःखेन with great difficulty, बुबुधे he identified.
Just as an unclear expression which has undergone certain changes in meaning due to lack of usage over a long period is understood by virtue of previous practice so also Hanuman had difficulty in recognising Sita who was devoid of any decoration.