Valmiki Ramayana Ayodhya Kanda Sarga 23 Shloka 25

प्रजापाल्यमनन्तरं afte ruling the subjects, वर्ष सहस्रान्ते after a thousand years, त्वयि you, वनवासं गते after you have withdrawn to the forest, आर्यपुत्राः your sons, करिष्यन्ति will rule them.

When after ruling the subjects for a thousand years you withdraw into the forest (for yativrata) your sons will rule them.