The story of Raksha Bandhan mentioned in Bhavishya Purana which refers to a Sura-Asur Yudh (war between Gods and demons). King Brutra, the powerful demon king, was attacking the Gods army with a furious approach and the Gods were on the edge of defeat.
Worried King, Indra, approached Guru Brihaspati, Guru of Gods, with a hope of finding a solution to stop the demon Brutra. Brihaspati advised Indra to get a sacred thread tied on his wrist on Shravan Purnima day. Sachi Devi, also known as Indrani, Lord Indras wife, sanctified a thread and tied on to his hand on Shravan Pournami. The sacred thread powered the Gods not only to get rid of the defeat but routed to a great victory for them.
Thus the sacred thread, also referred as Raksha, protected the Devas. Since then, every year on Shravan Purnima, Raksha Bandhan or Rakhi festival is celebrated.
Raksha Bandhan rituals & customs
Reference of Raksha Bandhan in History
Reference of Rakhi Festival in Mahabharat
Legend of Raksha Bandhan in Bhavishya Purana