What is the ruling on someone who said to his wife, whom he has not yet consummated the marriage with, "By divorce, I will not answer my wife if she calls me," then answered her, having forgotten what he said?

The majority of scholars hold that a divorce oath takes effect if the sworn-upon act occurs. However, Sheikh al-Islam Ibn Taymiyyah is of the view that it is an oath requiring an expiation for breaking an oath. As for if the sworn-upon act is done out of forgetfulness, the preponderant opinion is that divorce does not occur. This is the madhhab (definition) of Imam Shafi'i, a narration from Imam Ahmad, and the chosen view of Ibn Taymiyyah and Ibn al-Qayyim. This is further affirmed by Al-Nawawi and Al-Buhuti. Therefore, if you respond to your wife out of forgetfulness of the oath, there is nothing upon you, while emphasizing the prohibition of swearing by divorce.