百年好合

bǎiniánhǎo
may you live a long and happy life together (wedding greeting)

CEDICT

conjugal felicity; long-lasting harmonious union (of a couple) staying in love for the rest of their lives, for better or worse (usually said during a wedding ceremony)

Wiktionary

人夫感情长久不变通常作为结婚贺词

重編國語辭典

Entry Composition