A video of his 5th birthday party to be exact. I was watching it with Pixel.
And while he didn't say "peek-a-boo" in this particular video (didn't catch it anyways), the moment it was going Pixel was "peek-a-boo(ing)" profusely all throughout the video!
Link to that video
Link to a video where Marnie does say "peek-a-boo"
Parrot loves new bunny
(I'm linking to the videos instead of embedding them since they aren't mine.)