塞翁失马 (Sài Wēng shī mǎ)

Literally: The old frontiersman loses his horse

Example: “丢了工作后他开了自己的店，塞翁失马，焉知非福。” — Losing his job led him to open his own shop; it was a blessing in disguise.

English equivalent: A blessing in disguise

Origin: From the Huainanzi (2nd century BC), chapter Renjian: an old man on the frontier lost his horse, which came back leading a fine horse; his son broke a leg riding it, and the injury kept him out of a war in which most young men died. Each turn of fortune became its opposite. The full saying is 塞翁失马，焉知非福, "the old man lost his horse, but who knows if it was not a blessing?"