From Bai Juyi's (
白居易, Tang dynasty)
《长恨歌》, the closing lovers' vow between Emperor Xuanzong and Yang Guifei:
在天愿作比翼鸟,
在地愿为连理枝 - 'In heaven, let us be two birds flying wing-to-wing; on earth, let us be two branches intertwined.' The defining classical Chinese vow of undying love.