Neither the proverb nor the sentiment are particularly original, but I have no idea who said the words first, or how he might have said them. I do know the words are true. Courage is a form of tenaciousness, a refusal to quit when you want to quit because you're tired or humiliated or broken, and it is as necessary in everyday life as it is in moments of great upheaval. In fact, I could easily say that everyday courage is more important than the 'great deeds' sort, because every one of us will be in everyday situations, while not all of us will be called upon in our lifetimes to perform great deeds.
Courage is as essential to the writer as oxygen, no more and no less.
The writer who lacks courage will never succeed. I can't think of a safer sort of work. Let me define what writing is for you. You're going to attempt to sell the products of your mind to a world that doesn't care right now whether you breathe or not. While you are reaching out to editors, agents and publishers, you're going to fail.