SB 9.11.18
tata urdhvam brahmacaryam
dharyann ajuhot prabhuh
trayodasabda-sahasram
agnihotram akhanditam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
After mother Sita entered the earth, Lord Ramacandra observed complete celibacy and performed an uninterrupted Agnihotra-yajña for thirteen thousand years.