Once upon a time during sunset, when Lord Hanuman was busying in his diety’s pooja i.e. Lord Ram near Ram Setu, Sani Dev passed by. That time whenever Sani Dev use to visit anyplace, all the devtas use to bow in front or leave the place. But Lord Hanuman was too much focused & did not see Shani Dev coming.
With this attitude, Shani thought when all devtas give me respect, then why is this “Vanar” (money) not doing the same. He went to Lord Hanuman & said I’m the powerful Son of Sun, Shani Dev & I challenge you to fight with me. Daily during sunset Lord Hanuman use to take “parikarma” (round) of the Ram Setu, he replied the same to Shani Dev and requested him to go somewhere else. But Shani Dev was not listening. Shani Dev held the hand of Lord Hanuman & challenged him to fight with him.
On number of refusals by Lord Hanuman, when Shani Dev did not listen, Lord Hanuman tied Shani Dev in his tail. Shani Dev tried everything but could not loose the grip. Lord Hanuman started flying and while flying Shani Dev was tied with the tail, hitting the trees & mountains. After the parikarma, the body of Shani Dev was badly wounded and there was blood coming out from everywhere. Shani Dev requested Lord Hanuman to release him from the grip.
On a condition that Shani Dev will not give malefic effects to the followers of Lord Hanuman, Hanuman Ji released Shani Dev. Now because there were lots of wounds, Lord Hanuman also gave Shani Dev mustard oil to put on wounds. On this gesture Shani Dev said, anyone who will donate Mustard oil on Saturday, I will take away all his/her pains.