Valmiki Ramayana Ayodhya Kanda Sarga 23 Shloka 26
Original Shlokaपूर्वं राजर्षिवृत्त्या हि वनवासो विधीयते। प्रजा निक्षिप्य पुत्रेषु पुत्रवत्परिपालने।।2.23.26।।
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पुत्रेषु in sons, प्रजाः subjects, पुत्रवत् like sons, परिपालने in ruling, निक्षिप्य after entrusting, वनवासः retiring into the forest, पूर्वं राजर्षिवृत्त्या as rajarsis did in ancient times, विधीयते हि is established.
Leaving their sons to look after the subjects as their own children, the rajarsis in ancient times used to retire into the forest as per practice.