Shiva Purana
ब्रह्माणां च समुद्दिश्य वर्षाणां च सहस्रकम् ॥ मनसा ध्यानमाश्रित्य तपश्चक्रे महत्तदा॥ २२॥
With Brahmā as the goal of his meditation he performed a great penance for a thousand years.
english translation
brahmANAM ca samuddizya varSANAM ca sahasrakam ॥ manasA dhyAnamAzritya tapazcakre mahattadA॥ 22॥
hk transliteration by Sanscript