According to the tag on pixiv, the lyrics are from Mata ashita/See You Tomorrow, Madoka's character song which was used as the unaired ending of episodes 1 and 2 of the anime. I hope I copied the correct translated lyrics, but corrections are welcome if necessary. :P
Saying, "See you later,"I wave my handCracking a smile,yet I'm feeling lonelyThe truth is, I still have more to talk about"See You Tomorrow"