A 28-year-old man who paid for a Californian mother’s groceries died in a car accident less than 24 hours after performing the generous deed.

According to a Facebook post, after forgetting her debit card at home, Jamie-Lynne Knighten was close to asking the supermarket to hold her groceries. That is, until a kind stranger known as Matthew Jackson stepped in, politely offering to pay the $US200 bill.

“May I?”.. I’m overwhelmed and didn’t think I heard him correctly so he repeats “May I? May I take care of your groceries?” wrote Ms Knighten in a Facebook post.

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“I stutter and say “Oh, thank you.. How sweet, but no thank you, you don’t have to do that..”.

“So he calmly repeats himself again.. This time I look at him and he says “I would be glad to take care of your groceries as long as you promise to do it for someone else..” and I came to realize how much it would mean to him if I humbly accepted. So I did.”

After getting the man’s name, Ms Knighten said she rang Mr Jackson’s work to tell his boss about the good deed. She also hoped to send him a thank you card and a small gift.

But on the phone, she received some unexpected news.

“I hear crying on the other end of the line and my heart sinks … I just knew something was wrong,” Ms Knighten said.

The man’s boss told Ms Knighten that Mr Jackson was killed in a car accident the night after performing the random act of kindness.

“Not even 24 hours after meeting him. Gone. Just like that. How? Why? I don’t understand,” she said.

“He was a year younger than me and engaged to be married.

“His boss explained to me how amazing this young man was … what he did for me was just who he was as a person.”

Ms Knighten said the Facebook post was written as a tribute to the generous stranger.

“I am trying to make sense of something that will never be fully understood,” she said.

“I thought for sure I would get the chance to see him again, give him a hug and thank him at least once more in person. Now I won’t get that chance, but more importantly no one else will get the chance to meet him. And that breaks my heart.”

Facebook and Twitter tribute accounts titled Matthew’s Legacy were organised to honour Mr Jackson and “spread hope and kindness to everyone”.

Mr Jackson’s mother LeeAnn Krymow told the San Diego Union-Tribune that her son always had a generous personality.

“I knew my boy was like this,” Mrs Krymow said.

“He loved to be kind. He was just a really special kid. So cute, so intelligent, so talented, an accomplished musician. You wonder why these things happen.”