Hanuman was greatly devoted to Lord Rama. A particular incident was when Sita put sindoor on her forehead, Hanuman asked her why. To this, she replied that since she is the wife and companion of Lord Rama, the sindoor was a sign of her unconditional love and respect. Hanuman then covered his entire body with sindoor to prove his love for Lord Rama. Lord Rama was really impressed by this and granted a boon that those who worshiped Lord Hanuman in the future with sindoor would see all their difficulties go away.
One day Sita mata was wearing a red powder in the parting of her hair. Curiously Hanuman asked Sita mata that why she was wearing the vermillion powder. Sita mata replied “Hanuman! This is sindoor. Sindoor makes Lord Rama happy and would bless him with a long and prosperous life.” Hanuman ji thought that Sita ji loved lord Rama and she expresses her love by applying the sindoor.
Hanuman disappeared from the place and came back with his body fully covered with red sindoor. He covered his entire body, even his clothes and hair with sindoor. It looked as if Hanuman ji has taken a dip in the red river.
Sita ji looked at him in surprise and asked the reason for this act. He replied that if a little sindoor in her hairs can make Lord Rama happy then he being a daas has to do more to please his Lord. “I thought if just a pinch will do so much, what will happen if I cover my entire body with sindoor.”
Thus, Lord Ram gave a boon to all his devotees that those who apply sindoor to Hanuman and observe hanuman puja with sindoor would be graced with happy long life and all their wishes will fulfill.