Mahabharata 12.127.4

षष्टिं वर्षसहस्राणि सोऽतप्यद्गौतमस्तपःतमुग्रतपसं युक्तं तपसा भावितं मुनिम्

ṣaṣṭiṃ varṣasahasrāṇi so'tapyadgautamastapaḥtamugratapasaṃ yuktaṃ tapasā bhāvitaṃ munim

Our plain-language meaning of this verse is waiting for scholar review.