Valmiki Ramayana Bala Kanda Sarga 24 Shloka 9
Original Shlokaतस्मात्सुस्राव सरसस्सायोध्यामुपगूहते । सर प्रवृत्ता सरयू: पुण्या ब्रह्मसरश्च्युता।।1.24.9।।
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तस्मात् सरस: from that Manasa lake, सुस्राव flowed, सा that river, अयोध्याम् Ayodhya, उपगूहते surrounds, सर प्रवृत्ता because it flows from that lake, सरयू: is known as Sarayu, ब्रह्मसरश्च्युता released from the lake of Brahma, पुण्या sacred.
This river which flows from Manasa Sarovara is, therefore, known as 'Sarayu'. It surrounds Ayodhya. This sacred river flows from that lake of Brahma.