Valmiki Ramayana Yuddha Kanda Sarga 70 Shloka 35

तदा then, कपिः monkey, तत् that, शिखरम् top of mountain, व्यर्थम् ineffective, दृष्टवा seeing, तस्मिन् over him, रणे battle, रावणस्य Ravana's, सुतंप्रति over the son, द्रुमवर्षम् rain of trees, विससज्ज discharged.

Then seeing that the top of the mountain hurled was ineffective in that battle over Ravana's son, Hanuman discharged rain of trees.