Chaucer supposed that he was in company with a party of pilgrims going to Canterbury to pay their devotions at the shrine of Thomas à Becket. The party assembled at an inn in Southwark, called the Tabard, and there agreed to tell one tale each, both in going and returning. He who told the best tale was to be treated with a supper on the homeward journey. The work is incomplete, and we have none of the tales told on the way home.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894