What is the ruling of the Shari'ah concerning someone who said to his wife, "You are forbidden to me for what I have done," intending it as an oath, thinking he had not done something, and then doubted whether he had done it or not?

Your statement to your wife: "You are unlawful to me for what you have done," intending it as an oath to deny something she did not do, in your estimation, and you did not intend by this oath divorce (talaq) or a declaration of unlawful assimilation (zihar), then you are required to offer an expiation for a broken oath (kaffarat yamin). This is the view of Imam Al-Shafi'i, and it is the sounder of the two opinions if one did not intend anything specific.