Valmiki Ramayana Ayodhya Kanda Sarga 86 Shloka 12
Original Shlokaमहता तपसा लब्धो विविधैश्च परिश्रमैः। एको दशरथस्यैष पुत्रस्सदृशलक्षणः।।2.86.12।।
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एषः this Rama, महता by great, तपसा with austerities, विविधैः by various, परिश्रमैश्च with efforts as well, लब्धः was obtained, दशरथस्य Dasaratha's, एकः one, सदृशलक्षणः one with very similar virtues, पुत्रः is the son.
Rama is the only one among the sons who resembles his father in various virtues. Dasaratha obtained him after a great deal of austerities and efforts as well.