Valmiki Ramayana Ayodhya Kanda Sarga 55 Shloka 4
Original Shlokaगङ्गायमुनयो स्सन्धिमासाद्य मनुजर्षभौ। कालिन्दीमनुगच्छेतां नदीं पश्चान्मुखाश्रिताम्।।2.55.4।।
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मनुजर्षभौ two (best) among men, गङ्गायमुनयोः सन्धिम् the confluence of Ganga and Yamuna, गत्वा having reached, पश्चान्मुखाश्रिताम् flowing westward, कालिन्दी नदीम् river Kalindi (Yamuna), अनुगच्छेताम् proceed along.
Reaching the confluence of rivers Ganga and Yamuna, O Rama and Lakshmana best among men proceed along the Kalindi (Yamuna) river flowing westward.