Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 36
Sanskrit:
एवं वां तप्यतोस्तीव्रं तप: परमदुष्करम् ।दिव्यवर्षसहस्राणि द्वादशेयुर्मदात्मनो: ॥ ३६ ॥
ITRANS:
evaṁ vāṁ tapyatos tīvraṁtapaḥ parama-duṣkaramdivya-varṣa-sahasrāṇidvādaśeyur mad-ātmanoḥ
Translation:
Thus you spent twelve thousand celestial years performing difficult activities of tapasya in consciousness of Me [Kṛṣṇa consciousness].