After crossing the sea and killing the she-demon, Sihikaa, Hanuman decided to enter the heavily guarded city of Lanka to search for Sita.
However, a very powerful demon by the name of Lankini always guarded the city. When she saw Hanuman approaching the gates of the city, she stopped him. Hanuman insisted on going in but she punched him and Hanuman punched her back.
Lankini was surprised at the monkey's strength and looked at him closely with a puzzled look on her face. At once, she recognised him. She said, "Brahma had predicted that the day a monkey hits you, know that Ravana and all the other demons would be killed."
She set Hanuman free and allowed him to enter Lanka.