Shiva Purana
ध्रुवो वर्षसहस्राणि त्रीणि दिव्यानि कानने ॥ तपस्तेपे स बालस्तु प्रार्थयन्स्थानमव्ययम् ॥ ९ ॥
Even as a child Dhruva performed penance for three thousand divine years wishing for an imperishable status.
english translation
dhruvo varSasahasrANi trINi divyAni kAnane ॥ tapastepe sa bAlastu prArthayansthAnamavyayam ॥ 9 ॥
hk transliteration by Sanscript