Help me place this sample!

[everlastingRAINBOW]

So, I was watching a ballet the other day which features Tchaikocsky's Serenade for strings, and I went, "This is from an Ayu song!"

But which one? (Could very well be another Jpop song, but I think Ayu sampled it). I have listened to a good amount of her classical stuff and cannot place it: can you?

https://www.youtube.com/watch?v=xsGRglp6tvs

Thanks!