"For they were all of the same mind, that is, to be superior to others in the use of weapons, and they knew that one would not have the essential skills when they were needed unless they were practiced in advance.
The boxer cannot step into the ring with confidence if he has never received a blow. He who has seen his blood flowing, who has felt his teeth cracking under an opponent's blow, who has lain on the ground with his enemy over him, and still has not lost his courage; he who has been thrown to the ground time after time, only the more staunchly to stand up again - he may go into battle with high hopes.
For virtue grows when it is irritated, but a soul that gives in to fear has only fleeting glory. Blameless is he who, too weak to bear the burden, nevertheless hastens eagerly to assume it.
The reward for hours of toil waits where the temples of victory stand."
-- Roger of Hoveden, English chronicler, commenting on the training of Godfrey, Duke of Brittany, son of King Henry II. Maxims are borrowed from Seneca.
I have seen that quote in different forms from quite a few websites and books.
Usually "boxer" is replaced with "knight".