Valmiki Ramayana Bala Kanda Sarga 46 Shloka 6

त्वम् you, शुचि: भविष्यसि यदि if you remain pure and clean, वर्षसहस्रे thousand years, पूर्णे after completion, मत्त: through me, त्रैलोक्यभर्तारम् lord of three worlds, पुत्रम् son, जनयिष्यसि will give birth.

"If you remain pure and clean, on completion of a thousand years you will give birth to a son through me who will be the lord of the three worlds".