Valmiki Ramayana Sundara Kanda Sarga 6 Shloka 1

कामरूपधृत् who could assume any form, सः that (Hanuman), विमानेषु in the tall mansions, निकामम् freely, निषण्णः sad, पुनः again, लाघवेन rapidly, समन्वितः endowed with strength, लङ्काम् Lanka, विचचार started ranging

The mighty Hanuman, who could assume any form at will, not finding Sita in the tall mansions, again started rapidly ranging in Lanka. Hanuman enters the palace of Ravana -- description of the palace