What are the consequences for a man who said to his wife, "You are forbidden to me between me and my Lord," and then had intercourse with her the following day without either of them knowing the impermissibility of that?

Prohibiting one's wife [from oneself] takes effect according to the husband's intention. If he intended divorce, then it becomes a revocable divorce, provided it does not complete the three [divorces], and sexual intercourse is considered a return [to the marriage]. If he intended ẓihār (a pronouncement comparing his wife to an unmarriageable female relative), then sexual intercourse is not permissible before expiation, but sexual intercourse before it is excused due to ignorance of the ruling. If he intended an oath by Allah or did not intend anything, then an expiation for an oath becomes binding upon him, and delaying it is excused due to ignorance.