When My Girlfriend's Fitness Pants Disappeared

Please Help, Book Friends, Please Help Me Find This Book. I Only Remember This Sentence in the Book.

Little did she know that time is ruthless. Fifty years had passed when they met again. She had changed from a young girl to a gray-haired old woman, but he remained the same as before. She knew that there was a difference between immortals and mortals. Looking at this familiar face, she thought for a long time.