Valmiki Ramayana Aranya Kanda Sarga 32 Shloka 17

धीरः steadfast, यः who, पुरा in the past, महावने in a dense forest, दशवर्षसहस्राणि ten thousand years, तपः penance, तप्त्वा after performing, स्वयंभुवे to the Creator, शिरांसि heads, उपजहार offered.

This steadfast Ravana who had in the past performed penance for ten thousand years in a dense forest offered his heads to the selfborn Brahma.