Valmiki Ramayana Yuddha Kanda Sarga 20 Shloka 1

ततः then, वीर्यवान् heroic one, शार्दूलोनाम: by name Shardula, राक्षसः Rakshasa, आभ्येत्य: entering into, निविष्टाम् encamped, सुग्रीवेण: by Sugriva, अभिपालिताम् under the command of, ध्वजिनीम् bearing a flag as a mark, ददर्श: saw.

Then the heroic Rakshasa called Shardula entered into the area encamped by the army under Sugriva's command and saw the sign flag of the army. Ravana sends Suka as an envoy to Sugriva knowing that the Vanara army is advancing to Lanka. Vanaras capture Suka and Rama prevents the Vanaras from killing him and sets him free.