Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 63: Viśvāmitra Falls Prey to Lust
Text 1.63.26

एवं वर्षसहस्रं हि तपो घोरमुपागमत्॥

evaṁ varṣa-sahasraṁ hi tapo ghoram upāgamat

evam = he; varṣa-sahasram hi = for a thousand years; tapaḥ ghoram = in terrible austerities; upāgamat = thus engaged.

He thus engaged in terrible austerities for a thousand years.