From Musashi:

"The primary thing when you take a sword in your hands is your intention to cut the enemy, whatever the means. Whenever you parry, hit, spring, strike or touch the enemy’s cutting sword, you must cut the enemy in the same movement. It is essential to attain this. If you think only of hitting, springing, striking or touching the enemy, you will not be able actually to cut him. More than anything, you must be thinking of carrying your movement through to cutting him.”

Pretty sure it's mentioned in Twelve Virtues of Rationality, which provides a decent summary in context -- although you should read Musashi's Book of Five Rings if you really want to absorb the concept. He's a lucid (if sometimes infuriatingly vague) writer, and there are several good translations floating around.

I think this is one of the earlier mentions.