新人如故白头偕老
[xīn rén rú gù bái tóu xié lăo]
It expresses the hope that even though they are new people (a new couple), they feel as close and familiar ('如故') to each other as old friends. They will love each other until their hair turns white from old age - an idiom for staying together forever, representing eternal love and deep affection for long term relationships.