The 39-year-old "Wild" star took to Instagram and posted a video with her two-year-old son lip-syncing Raffi's "Bananaphone", reported Entertainment Weekly.
"Good morning! Pick up the banana phone! #toddlertime#funwithfruit" she wrote alongside the video.
"Bananaphone" is a popular children's album released by Raffi and Michael Creber in 1994.
The title track's lyrics describe the bananaphone and uses many puns including "It's a phone with appeal!" (a peel) as Raffi extols the virtues of his unique telephone.
Witherspoon was last seen in "Hot Pursuit' which also starred Sofia Vergara.