Valmiki Ramayana Ayodhya Kanda Sarga 35 Shloka 9
Original Shlokaयथावयो हि राज्यानि प्राप्नुवन्ति नृपक्षये। इक्ष्वाकुकुलनाथेऽस्मिं स्तल्लोपयितुमिच्छसि।।2.35.9।।
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नृपक्षये after the death of the king, यथावयः according to the age, राज्यानि kingdoms, प्राप्नुवन्ति हि obtain, अस्मिन् in this, इक्ष्वाकुकुलनाथे lord of Ikshvaku dynasty (Rama), तत् that one, लोपयितुम् to break, इच्छसि wish.
After the death of the king, the princes will obtain the kingdom according to seniority. Why do you want to break this tradition in the case of Rama, the lord of the Ikshvaku dynasty?