Keep on going, and chances are you will stumble on something, perhaps when you are least expecting it. I never heard of anyone stumbling on something sitting down.

Charles F. Kettering (1876-1958) American inventor, engineer, researcher, businessman

(Attributed)

Variant:

"Keep on going and the chances are you will stumble on something, perhaps when you are least expecting it. I have never heard of anyone stumbling on something sitting down."

I am unable to find this precise phrase in Kettering's writings or primary writings about him. However, this appears to have been part of a common set of phrases he used, which indicates either it is accurate or a paraphrase of one of them:

Accomplishment has been an accident as a rule. Nobody ever stumbled while he was standing still. You only stumble when you are moving. So we always had it a rule in our organization when we lacked intelligence we speeded up motion, because the chances of stumbling infinitely increased.
"250 at Luncheon Honor Kettering," New York Times (1936-11-11)

Nobody ever found anything while sitting down. So, Q.E.D., don't be afraid to stumble.
“Don’t Be Afraid to Stumble,” The Rotarian (1952-01)