Page "Ravana" Paragraph 96

According to legends, when Sita's swaymvar was introduced Ravana also had gone and he came before Lord Rama but some of the officials and on Sita's instructions they told him that his sister was being kidnapped. On hearing this Ravana immediately departed from the swaymavar and arrived at his castle only to found that her sister was well and sound.

On returning this Sita's swaymvar he saw that she was already engaged.

This led to a great outrage in Ravana and threatened Sita that one day she will be his wife.