Valmiki Ramayana Bala Kanda Sarga 18 Shloka 21
Original Shlokaअतीत्यैकादशाहं तु नामकर्म तथाऽकरोत्। ज्येष्ठं रामं महात्मानं भरतं कैकयीसुतम्।।1.18.20।। सौमित्रिं लक्ष्मणमिति शत्रुघ्नमपरं तथा। वसिष्ठ: परमप्रीतो नामानि कृतवान् तदा ।।1.18.21।।
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तथा and, एकादशाहम् eleven days, अतीत्य having completed, नामकर्म naming ceremony, अकरोत् he (Vasishta) performed, महात्मानम् illustrious one, ज्येष्ठम् eldest son, रामम् as Rama, कैकयीसुतम् the son of Kaikeyi, भरतम् as Bharata, सौमित्रिम् a son of Sumitra, लक्ष्मणमिति as Lakshmana, अपरम् the other son, शत्रुघ्नम् as Satrughna, परमप्रीत: highly delighted, वसिष्ठ: Vasishta, तदा then, नामानि कृतवान् gave names.
After a lapse of eleven days, the naming ceremony was performed. Highly delighted preceptor Vasishta named Kausalya's son Rama, Kaikeyi's son Bharata, one son of Sumitra, Lakshmana and the other Satrughna.