Valmiki Ramayana Ayodhya Kanda Sarga 54 Shloka 6
Original Shlokaनूनं प्राप्ताः स्म सम्भेदं गङ्गायमुनयोर्वयम्। तथा हि श्रूयते शब्दो वारिणो वारिघट्टितः।।2.54.6।।
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वयम् we, गङ्गायमुनयोः Ganga and Yamuna, सम्भेदम् confluence, प्राप्ताः स्म have reached, नूनम् it is certain, तथा हि as for, वारिघट्टितः dashed by waters, वारिणः waters, शब्दः sound, श्रूयते is heard.
We have surely reached the confluence of Ganga and Yamuna which can be ascertained from the sounds of clashing water.