Valmiki Ramayana Ayodhya Kanda Sarga 110 Shloka 27
Original Shlokaभगीरथात्ककुत्स्थस्तु काकुत्स्था येन विश्रुताः। ककुत्स्थस्य च पुत्रोऽभूद्रघुर्येन च राघवाः।।2.110.27।।
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भगीरथात् from Bhagiratha, ककुत्स्थ: Kakutstha, येन by whom, काकुत्स्थाः as Kakutsthas, विश्रुताः are celebrated, ककुत्स्थस्य च to Kakutstha, रघुः Raghu, पुत्रः son, अभूत् was born, येन by whom, राघवाः Raghavas
To Bhagiratha was born Kakutstha, by whom your race is wellknown. A son Raghu was born to Kakutstha and hence all of you are known as Raghavas.