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Is there a proverb that fits the following situation:

John and Anderson (while walking):

John: [sees a $100 bill lying in the road, and shouts] Yahoo! $100!

Anderson: [takes the money and puts it into his pocket]

Reflection: John exclaimed when he saw the money in the road, but he didn't try to take it. Anderson was smarter, as, when he saw John exclaiming, he took the money and put it into his pocket. Consequently, John now repents his foolish excitement.

My question is, is there a proverb for such situations?

EDIT: When I asked my friend, he suggested the following saying:

"Some feel the rain others just get wet."

you snooze, you lose

informal
​if you do not pay attention and do something quickly,
someone else will do it instead of you:
Cambridge Dictionary

If you wait too long to do something, that opportunity might become
unavailable.
Farlex Dictionary of Idioms

John: Yahoo! $100!
(Anderson takes the money)
John: Hey, I saw it first!
Anderson: You snooze, you lose.

There are several possible sayings:

Actions speak louder than words.

[Cambridge Dictionary]

said to emphasize that what you do is more important and shows your intentions and feelings more clearly than what you say

The early bird catches the worm.

[Cambridge Dictionary]

said to advise someone that they will have an advantage if they do something immediately or before anyone else does it

He who hesitates is lost.

[Merriam-Webster]

—used to say that it is important to make decisions and do things in a quick and definite way

// I took my time and when I got to the store, they were all sold out. I guess "he who hesitates is lost."

Don't count your chickens before they are hatched.

It means, don't celebrate a victory that hasn't happened yet. It comes from the fact that not every chicken egg hatches out a chick. So if you count your future chickens from the number of eggs you have, you'll likely be overestimating.

It's very common in the US, and often used for just this type of situation. Also, with the same meaning (and likewise bird-themed)

A bird in the hand is worth two in the bush.

In this case, the $100 is the bird in the hand. $100 in your hand is worth $200 on the ground...

While the other answers dealt with one perspective (quite effectively at that) of the given situation, it appears that the OP's emphasis is on John's repentance.

Consequently, John repents over his foolish excitement.

In hindsight, John thinks he should not have shown his excitement (and that would have given him a better chance of grabbing the cash first, assuming Anderson did not see it yet). In other words, he should have kept it close to his chest.

TFD (idioms):

keep (something) close to (one's) chest

To keep one's plans, intentions, or tactics secret from everyone else.

Refers to holding one's playing cards close to one's chest in a card
game, so as not to allow other players to see one's hand.

We're all
curious about what the boss has been discussing in those meetings with
the lawyers, but she's keeping it close to her chest.

Sorry for not
being more straightforward about my plans, but I'm keeping this one
close to the chest for the time being.

Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

Strike while the iron is hot.

This proverb advises you to act decisively and take advantage of an opportunity when it first presents itself. By waiting (as slow-witted John did) you risk losing your chance.

The meaning is derived from the work of a blacksmith. In order to shape iron, the smith has to strike it while it is glowing hot and malleable. If he waits too long and it cools down, it can’t be hammered or bent into a new shape. His opportunity has been lost.

John was wise. Anderson was smart. The U.S. mint was genius.

The meaning is derived from the work of a blacksmith. In order to shape iron, the smith has to strike it while it is glowing hot and malleable, clicker heroes 2.0