Hanumanji said:
संकर सहस बिष्नु अज तोही। सकहिं न राखि राम कर द्रोही॥ (4)
Listen, O ten-headed Ravana, I tell you on oath: there is none to save him who is opposed to Sri Rama. Shankara, Vishnu and Brahma in their thousands are unable to protect you, an enemy of Sri Rama. (Sundarkand)
Some say Ravana already knew this. Is it true? Maybe it is given in some Purana