Valmiki Ramayana Sundara Kanda Sarga 57 Shloka 9

तदा then, विविधाभ्रघनासन्नगोचरः passing through the clouds clad in white, धवळाम्बरः white sky, दृश्यादृश्यतनुः appearing and disappearing, वीरः hero, अम्बरे in the sky, चन्द्रायते like the Moon.

Passing through the clouds, appearing and disappearing, the heroic Hanuman clad in white was like the Moon in the white sky.