"Now listen to my story, yes listen while I sing Of days of old in England, when Arthur was the king, Of Merlin the magician, and Guinevere the queen, And Lancelot, the bravest knight the world has ever seen.

In days of old, when knights were bold, this story’s told Of Lancelot.

He rode the wilds of England, adventures for to seek, To rescue maidens in distress and help the poor and weak. If anyone oppressed you, he'd be your champion. He fought a million battles, though he never lost a one.

In days of old, when knights were bold, This story’s told of Lancelot. In days of old, when knights were bold, this story’s told Of Lancelot."

From an old TV show, you can listen to it here.

No idea whether that's the origin of the line, but it's the oldest occurrence I managed to find.