白首若相离此生若不弃
[bái shŏu ruò xiāng lí cĭ shēng ruò bù qì]
Literal translation: If we gray together (old age together / growing old together), then this life will not abandon you. This conveys undying commitment to the partner throughout the life until the end, and beyond. It expresses unwavering loyalty and everlasting love towards someone.