Story of Ganpati, demon Analasura and durva grass (karuka mala) is found in the Puranas. When ignorance overcomes divinity, Analasuran thinks himself to be the most powerful. Analasura became strong and powerful and started harassing all living beings on earth.
Devas, Rishis, humans and all other living beings in the universe approached Shiva and asked to put an end to the rule of Analasura on earth.
Shiva asked them to approach Ganpati as only he had the power to annihilate Analasuran.
The living beings then approached Ganpati, who readily agreed to put an end to the tyranny of Analasuran.
Ganpati asked Analasura to stop all Adharmic activities. But the demon did not listen to Ganpati and challenged him to battle.
It was impossible to defeat Analasura using weapons. He was invincible to weapons.
Ganpati then thought of idea and challenged Analasuran for a fist fight.
While the wrestling was underway, Ganpati opened his big mouth and swallowed Analasur. The agni in the stomach of Ganpati killed Analasuran.
All living beings were happy but Ganpati showed signs of uneasiness due to indigestion.
Sage Kashyapa then offered 21 durva grass to Ganpati.
Ganpati ate the durva grass and immediately it helped in curing his indigestion and stomach ailments.
Ganpati then told the assembled crowd that his blessings will always be there with who offers him durva grass.