What is the expiation for one who swore by the limit of Allah regarding eating and drinking, and then ate and drank?

The lawful oath that establishes a vow and entails expiation is an oath by Allah Almighty, or by one of His names, or one of His attributes. "Hudud Allah" (the boundaries set by Allah) is none of these; therefore, it is not permissible to swear by it, nor does an oath become established by it, nor does expiation become obligatory for breaking it.