Valmiki Ramayana Aranya Kanda Sarga 11 Shloka 12
Original Shlokaस हि तेपे तपस्तीव्रं माण्डकर्णिर्महामुनिः।।3.11.12।। दशवर्षसहस्राणि वायुभक्षो जलाश्रयः।
1 source checked for this shloka
महामुनिः the great sage, सः that, माण्डकर्णि: Mandakarni, वायुभक्षः living on air, जलाश्रयः standing in water, दशवर्षसहस्राणि for ten thousand years, तीव्रम् intense, तपः penance, तेपे performed.
Great sage Mandakarni standing in water and living on air performed rigorous penance for ten thousand years.