Saint George was a knight and on a time he came to a city which is called Silene. And by this city was a sea, wherein was a dragon which envenomed all the country.
And when he came nigh the city he venomed the people with his breath, and therefore the people of the city gave to him every day two sheep for to feed him, because he should do no harm to the people, and when the sheep failed there was an ordinance made in the town that there should be taken the children and young people of them by lot.
So it happed that the lot fell upon the king's daughter. Then did the king array his daughter like as she should be wedded, and led her to the place where the dragon was.
When she was there Saint George passed by, and when he saw the lady he demanded of the lady what she made there and she said: "Go ye your way fair young man, that ye perish not also."
Then said he: "Tell to me what have ye and why weep ye, and doubt ye of nothing."
When she saw that he would know, she said to him how she was delivered to the dragon. Then said Saint George: "Fair daughter, doubt ye no thing hereof for I shall help thee."
She said: "For God's sake, good knight, go your way, and abide not with me, for ye may not deliver me."
As they spake together the dragon appeared and came running to them, and Saint George was upon his horse, and drew out his sword and garnished him with the sign of the cross, and rode hardily against the dragon which came towards him, and smote him with his spear and hurt him sore and threw him to the ground.
–– The Golden Legend or Lives of the Saints, (compiled by Jacobus de Voragine, translated by William Caxton, excerpt edited and abridged by Bard)
The complete original text can be found at Medieval Sourcebook.