Jennifer Garner and Ben Affleck's Son Left the Sweetest Note in a Library Book -- See the Pic!

Jennifer Garner Palos Verde Premiere
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Their little boy, Samuel, is all about spreading love and positivity!

If there’s an award for sweetest 5-year-old on the planet, it should go to Jennifer Garner and Ben Affleck’s son, Samuel!

Garner took to Instagram on Monday to share a photo of an adorable handwritten message that the young boy left in a library book.

The heartwarming note read, “Hello, you are loved. I believe in you,” and had a small black heart drawn onto the bottom. The discovery was made by a librarian, who informed Garner of the sweet gesture.

“When the librarian finds a love note your son tucked into a picture book...♥️♥️♥️♥️. #actsofkindness #spreadlove #belikemlkjr #lovenote,” the proud mom captioned the pic.

When she's not busy with her kids -- she's also mom to daughters Violet, 12, and Seraphina, 9 -- Garner has her pets to keep her busy, frequently sharing the fun on Instagram.

In December, the 45-year-old actress shared a video showing her reading to her pet chicken, Regina George. Then on Saturday, she baked with her golden retriever, Birdie.

Watch Garner talk to ET about spending holidays with Affleck and their kids in the video below.