A young mum was too scared to open her own front door to a stranger; now she has been overwhelmed by the selfless reason the young man came knocking.

Melissa Vang was in her home in Sacramento, California with her two young children when she heard someone on her front porch.

She feared the worst, and decided to ignore it, ABC10 reports.

But after reviewing the CCTV footage, she saw for herself why Tyler Opdyke was trying to capture her attention.

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In the video, posted on Facebook on September 16, Opdyke walks up to the camera, holds up a wallet, flips it over revealing a wad of cash, places it on the doormat and walks off.

She realised it was her husband's wallet after checking the video, which contained more than $1500 for a family feast they were planning.

The 18-year-old said he was "knocking on the door for a minute or two."

"I actually heard her little girls ... and was wondering 'why is nobody answering?'" he said.

"I'm like 'how can I find some way to let them know that I have this [wallet]?' I was like, 'perfect, here's a camera right there.'"

Opdyke was out dropping off flyers for his uncle's business in the neighbourhood when he found the wallet in a driveway.

He said that while the thought of all the money was exciting as he is saving for, he knew it wasn't his.

"I don't know their family, I don't know what they're going through," he told KXTV.

His younger cousin also threatened to dob him in if he didn't return it.

Footage of his good deed has since gone viral on Facebook with more than 45,000 views.

Vang said she managed to get in contact with the teen and give him a reward.

"I think we all need to be reminded that there are still good people out there," she posted.

"His act of kindness cannot go unrecognised.

"Thank you Tyler."