Srimad-Bhagavatam: Canto 9 - Chapter 1 - Verse 18
Sanskrit:
तत ऊर्ध्वं ब्रह्मचर्यं धार्यन्नजुहोत् प्रभु: ।त्रयोदशाब्दसाहस्रमग्निहोत्रमखण्डितम् ॥ १८ ॥
ITRANS:
tata ūrdhvaṁ brahmacaryaṁdhāryann ajuhot prabhuḥtrayodaśābda-sāhasramagnihotram akhaṇḍitam
Translation:
After mother Sītā entered the earth, Lord Rāmacandra observed complete celibacy and performed an uninterrupted Agnihotra-yajña for thirteen thousand years.