白头偕老 bái tóu xié lǎo
Meaning: To live together until the white hairs of old age; Especially related to wishing a newly wed couple a long and happy marriage.
白 bái - white (Phonetic: by)
头 tóu - head (Phonetic: toe)
偕 xié - together (Phonetic: she'air)
老 lǎo - old, aged, experienced (Phonetic: lou'd - as in the first syllable of loud)