Valmiki Ramayana Yuddha Kanda Sarga 51 Shloka 1

तदा then, तरस्विनाम् courageous, नर्दताम् increasing, तेषांवानराणाम् by those Vanaras, सुतुमुलम् tumultuous noise, शब्दम् sound, रावणः Ravana, राक्षसैःसार्धम् all Rakshasas, शुश्राव heard.

Then Ravana and all the Rakshasas heard the increasing and highly tumultuous sound made by the courageous Vanaras. Ravana comes to know that Rama and Lakshmana are released from the bondage. Disappointed by the news Ravana sends Dhumraksha to fight with Rama and Lakshmana.