Indra wanted to test Lakshmana, so he sent a beautiful apsara to tempt him. Lakshmana chased her away, but a few strands of the apsara's hair clung to Lakshmana's clothing.
That evening, Sita noticed the strands of hair and teased Lakshmana. "I see you've found some lovely lady here in the wilderness," she said, laughing. "I wonder what your Urmila would say about that?"
"How dare you doubt my fidelity!" shouted Lakshmana, and he jumped into their campfire.
The fire did not burn him. "You see?" said Lakshmana. "That is the proof that I am forever faithful to my Urmila."
Inspired by: Sita: An Illustrated Retelling of the Ramayana by Devdutt Pattanaik
Notes: This story is on p. 118 of the book. Pattanaik notes that this legend comes from the Baiga Ramayana of central India.