During the war against Ravana, Hanuman is sent to fetch the Sanjivani, a powerful life-restoring herb from the Dronagiri mountain in the Himalayas, to revive Lakshmana was severely wounded by Indrajit son of Ravana.
With the coming of Indrajit, Ravana's son, who had the ability to make himself invisible and who wielded terrible weapons of boon from the gods, the scenario was turning bad in Rama's camp. Even Lakshmana lay in a swoon. There remained only Rama, Hanuman and Vibheeshana to lead and not many beyond finger-count among the soldiers.
When everything seemed apparently lost, Jambavan asked Hanuman to bring some herbs from the Sanjeevani Mountain. So, with an urgent mission on hand, Hanuman leapt skyward and flew north like lightning. Having spotted the Sanjeevani, he alighted upon it, only to see that it was densely forested, difficult to distinguish one herb from another. Instantly he decided that instead of trying to discover the herbs needed, he would carry the mountain itself back to Lanka!
With the herbs extracted from Sanjeevani, Lakshman was recovered and killed Indrajit .