"He watched how polite you were to everyone."
The 10-year-old boy and his family sat behind the NFL star on a flight to Connecticut.
The boy had just made an elite hockey team and they were travelling for his training.
The boy's parents wanted to thank Sanu for being an inspiration to their athlete-in-the-making.
"You are an inspiration to children and for that you should be proud," the note said.
Sanu tweeted the note with the caption, "This definitely put a smile on my face."
It has been liked more than 10,000 times and retweeted more than 2700 times so far.
Sanu's team, the Atlanta Falcons, also weighed in on Twitter, calling Sanu a "role model both on and off the field".