Good question! Without looking anything up, there are two scenes that come to mind that it could be but my deduction is purely based on my own biases. First of the two would be the easy on, when he's talking to Jenny's grave and says "You died on a Tuesday...." The other would be the scene when Jenny introduces Forest to his son and he asks if he's smart. So, again, based on what I imagine of Tom Hanks's sensibilities with the way he played the scene, I'm going with the latter, when Jenny introduces Forest to his son. I'm looking forward to finding out the right answer!
While this is not the correct answer I will give you an upvote anyways for attempting. It was actually when he met Lt Dan again and said he had new legs. Ironic eh? :)