Book I - Chapter XIII - Of a dream of the King with the Hundred Knights.
So by Merlin's advice there were sent fore-riders to skim the country, and they met with the fore-riders of the north, and made them to tell which way the host came, and then they told it to Arthur, and by King Ban and Bors' council they let burn and destroy all the country afore them, there they should ride.
The King with the Hundred Knights met a wonder dream two nights afore the battle, that there blew a great wind, and blew down their castles and their towns, and after that came a water and bare it all away. All that heard of the sweven said it was a token of great battle. Then by counsel of Merlin, when they wist which way the eleven kings would ride and lodge that night, at midnight they set upon them, as they were in their pavilions. But the scout-watch by their host cried, Lords! at arms! for here be your enemies at your hand!