Valmiki Ramayana Ayodhya Kanda Sarga 73 Shloka 22
Original Shlokaसततं राजवृत्ते हि ज्येष्ठो राज्येऽभिषिच्यते। राज्ञामेतत्समं तत्स्यादिक्ष्वाकूणां विशेषतः।।2.73.22।।
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राजवृत्ते in royal custom, सततम् always, ज्येष्ठः the eldest son, राज्ये in the kingdom, अभिषिच्यते is consecrated, एतत् this, राज्ञाम् to all kings, समम् is the same, तत् that, इक्ष्वाकूणाम् for Ikshvakus, विशेषतः स्यात् especially it applies.
According to the established law of royalty, the eldest son is invariably crowned in the kingdom. This is true of all kings. This is a special tradition of the kings of the Ikshvaku race.